The Ultimate Guide to Outlook SMTP Settings in 2025

If you’re used to classic Outlook, navigating the new Outlook settings can be a real hassle. Microsoft changed its design completely. Configuring Outlook SMTP server settings, for example, is now different compared to before. 

But the thing is, every online tutorial still shows the classic interface, with screenshots that don’t match what’s on your screen. You end up right back at square one, wasting valuable time and risking misconfiguration that could lead to missed messages. I spent time clicking through menus so that you don’t have to.

In this article, I’ll show you different ways you could configure your Outlook SMTP settings. I’ll walk you through a step-by-step process, and by the end of it, you’ll be able to:

• Configure a third-party email client like Thunderbird with Outlook SMTP, POP3, and IMAP settings that allow it to send and retrieve messages from your Outlook.com account.
• Set up Outlook with another email account’s SMTP, POP3, and IMAP settings, so Outlook can handle sending and receiving messages on its behalf.

We’ll also go over the ways you could fix SMTP server issues, the limitations of Outlook SMTP, and an SMTP alternative if you plan on running cold outreach campaigns or mass email campaigns.

What is Outlook SMTP and how does it work?

Outlook SMTP is the protocol Microsoft uses to send emails from your Outlook.com account or a third-party email client.

When you configure an email client like Thunderbird with Outlook SMTP settings, you’re essentially proving ownership of your account. This usually requires your full email address and password. However, your regular password for logging into your Outlook account might not work. You’ll need an app password if you’ve set up two-step verification. 

Outlook then encrypts your message using STARTTLS (via port 587) to scramble its contents during transit, ensuring hackers can’t intercept it mid-journey. And finally, the SMTP server acts as a relay. It checks your credentials, hands off the email to Microsoft’s network, and routes it toward the recipient’s inbox.

One thing to note is that Outlook email SMTP settings vary slightly depending on whether you’re using a free Outlook.com account, Microsoft 365, or a third-party domain. For example, Microsoft 365 accounts often use smtp.office365.com instead.

Outlook SMTP vs POP3 vs IMAP – what is the difference?

In a nutshell, SMTP is the protocol used to send emails, while POP3 and IMAP are responsible for receiving them. However, the main difference between POP3 and IMAP is how they manage and store these emails.

POP3, for example, works on a download-and-delete model. When you use POP3 with Outlook, it connects to the server, downloads new emails to your device, and (by default) removes them from the server. This means you can access your emails offline but won’t see the same messages on another device since POP3 stores emails only on the device that downloads them.

However, there’s a workaround with Outlook. You can turn on the leave a copy of the message on the server option and this will make sure your emails stay on the server and are accessible from all your devices.

IMAP on the other hand, is a two-way synchronization protocol for receiving emails. Instead of removing messages from the server, IMAP leaves them there and syncs every action you take across all your devices. Whether you read, delete, or organize an email on your phone, those changes instantly reflect everywhere. 

However, while IMAP is ideal if you are using multiple devices,  it relies on server storage. So if your Outlook.com account runs out of space, you might stop receiving new emails until you clear some room.

Here are some key differences at a glance:

How do you set up Outlook SMTP, IMAP, and POP3?

Let’s get practical. In this section, we’ll look at two ways of configuring Outlook:

1. How to set up third-party email protocols in Outlook
2. How to set up Outlook SMTP, IMAP, and POP3 settings in a third-party email client

Let’s dive into each option.

How to configure third-party email protocols in Outlook

When done correctly, this will allow you to manage all your email accounts, even non-Outlook email accounts, directly within Outlook familiar interface. You won’t have to switch between different platforms anymore.

And Microsoft has actually made it easy to set this up. For demonstration purposes, I used Zoho Mail but you can follow the same steps regardless of the email client you’re using.

Step 1: Sign into your third-party email app and turn on SMTP, IMAP, and POP3. 

Check your email app’s help center for documentation if you need help finding the location of these settings. 

In my case, it’s under mail settings. But in general, you can look under settings and accounts.

Step 2: Sign into your Outlook account.

Now, click on the settings gear icon at the top right corner of the page.

Step 3:  Click on accounts

A pop-up window of the settings will open. Click on accounts and add an account.

Step 4: Write your email address in the box provided and click on continue. 

A pop-up window will appear where you’ll be asked to add your email account.

Step 5: Input your password in the box provided. 

Most email apps will provide you with an app password which you’ll use here instead of your email password.

Look into your email client’s documentation for help generating an app password. But in general, you might find it under security settings or two-factor authentication settings. 

Step 6: Toggle on the show more option. 

You’ll realize Outlook automatically filled in the SMTP and IMAP configuration settings of your third-party email app and the ‘use recommended settings’ option turned on.

Step 7: Confirm the SMTP and IMAP/POP3 configuration settings before proceeding.

One thing to note is that you have to set up an incoming protocol for receiving messages while setting up an SMTP server. By default, Outlook chose IMAP. But if you’d like to change to POP3, click on ‘select provider’ and choose the ‘POP’ option.

I confirmed these were the right settings Outlook filled in. Now, under SMTP password you can leave it blank as long as you put in your app password in the first box.

Step 8: Click on Continue and wait for Outlook to connect with your third-party email client.

Congratulations, you have successfully configured third-party SMTP and IMAP/POP3 settings in Outlook. You can now send and receive emails directly from your Outlook account.

How to configure Outlook SMTP, IMAP, and POP3 server settings in a third-party email client.

This allows you to send emails from your Outlook.com account using the interface of a third-party email client, such as Thunderbird.

Step 1: Enable POP and IMAP access in Outlook settings

Turning this on will allow your email client to connect to Outlook without blocking the connection. To do this:

• Log into your Outlook.com account on the web.
• Click on the settings gear at the top right corner of the page. 
• Under the settings click on Mail>Forwarding and IMAP.
• Toggle on the ‘let devices and apps use POP/IMAP’.

Step 2: Turn on two-step verification

This is important because you’ll need an app password when setting up Outlook SMTP in your preferred email app. And to generate an app password you have to turn on two-step verification.

To do this:

• Sign in to your Outlook.com account on the web and go to the security page.
• Click on ‘manage how I sign in’.

• Click on turn on two-step verification. You’ll be prompted to verify your identity. You can do so using an email address or an authenticator app. 

Step 3: Generate an app password

When you turn on two-step verification, app passwords will appear below the additional security section. Click on ‘create a new app password’ 

Write down the generated password. You’ll use it later on. But don’t worry you can still generate new passwords if the one you generated ever gets lost.

Step 4: Note down the email configuration settings

Write down the email settings so you have them ready for setup.

You’ll now use these settings in your third-party email client. Don’t worry, we’ll go through some configuration examples to see this in practice.

Outlook SMTP, IMAP, and POP3 configuration examples

Outlook SMTP servers are trusted by most email providers. This increases the likelihood that emails you send through Outlook will have a high email deliverability rate. For demonstration, we’ll look at two configuration examples:

Reply.io Outlook configuration

Reply.io is a cold email and multi-channel outreach tool with advanced tools like real-time B2B database, AI SDR, and outreach sequences.

By connecting Outlook SMTP, you can use Reply.io’s AI-powered features to find prospects and convert them into customers while maintaining the human touch of your Outlook domain.

Here’s how to go about it:

• In Reply.io, go to Settings > Email Accounts > New email accounts>Create manually
• Select Outlook from the list of email providers and you’ll be redirected to log into your Outlook account. Once you log in, your Outlook email will be added.

Thunderbird SMTP and IMAP configuration

Thunderbird lets you manage Outlook emails alongside accounts from Gmail, ProtonMail, or even self-hosted servers—all in one interface. But unlike Outlook design, it is highly customizable and supports add-ons. You can install extensions, create custom filters, and change the app’s theme.

Here’s how to connect Thunderbird with Outlook:

• Open the Thunderbird app.
• Click on ‘email’ to set up an account.

• Enter your full name, your Outlook email address, and the app password you generated earlier.
• Click on configure manually.
• Enter the Outlook SMTP and IMAP/POP3 settings (I chose IMAP as my incoming protocol but you can also choose POP3 from the drop-down menu).

How to fix common Outlook SMTP server issues and errors

As you set up Outlook email configuration, you might encounter various issues. I looked into some of the most common issues people raised at Microsoft support. Below are four common errors and their solutions:

Username and password not accepted

This error occurs when Outlook rejects your login credentials, even if they’re correct. It often happens because Microsoft requires an app-specific password for third-party clients if your account has two-factor authentication enabled.  

To solve this, generate an app password via your Microsoft account security page. Navigate to Security > Manage how I sign in > app password>create a new app password, then use this password—not your regular account password—in your email client’s SMTP settings for Outlook.

SSL/TLS handshake failed

This arises when your email client’s security settings conflict with Outlook requirements. For example, using SSL encryption on port 587 (which requires STARTTLS) or outdated TLS versions. To fix this, in your client’s SMTP settings, set encryption to STARTTLS (not SSL/TLS) for port 587. For port 465, use SSL/TLS instead.

Port 25 blocked by ISP

Many internet providers block port 25 (a legacy SMTP port) to prevent spam. If your client defaults to this port, emails fail to be sent. When you encounter this, try switching to port 587, which is unblocked and supports modern encryption. In your email client’s SMTP settings, manually set the port to 587 with STARTTLS encryption. 

Relay access denied

This simply means Outlook SMTP server refuses to send emails on behalf of another domain. Its servers are designed to send emails only from addresses linked to your Outlook/Microsoft account. For example, If your Outlook account is [email protected], you can’t use Outlook SMTP to send emails as [email protected] unless your business domain is explicitly authorized. 

For business emails, make sure your Microsoft 365 admin enables SMTP relay in the Exchange Admin Center under Mail Flow > Connectors. For personal accounts, use Outlook SMTP only with your Outlook-registered email address.

Outlook SMTP limitations

For casual use, Outlook SMTP works. But if you’re scaling outreach campaigns, it’s a straitjacket. It’s important to understand these limitations before you fully embrace Outlook email infrastructure. Here are some limitations you might encounter:

#1. Daily and monthly sending limits

For Microsoft 365 subscribers, you’re capped at 5,000 total recipients per day, with no single email going to more than 500 people. But even within that 5,000, only 1,000 can be first-time recipients—people who’ve never gotten an email from you before. 

Non-subscribers (free Outlook.com accounts) face stricter, undisclosed limits that tighten further if Microsoft suspects spam-like behavior. If for example, you’re blasting a promotion to 2,000 new leads. With Outlook SMTP, you’d need to split that over two days to stay under the “new recipient” ceiling. Push harder, and your account gets flagged.

#2. Message rate limit

Outlook restricts the number of emails you can send per minute to 30 messages. This throttling isn’t just about spam, it’s to prevent server overload. For example, if you’re using a CRM to automate follow-ups, a burst of 50 emails in a minute will see 20 rejected instantly. You’ll have to retry later, creating lag in time-sensitive campaigns.

#3. Attachment size limitations

Attachments max out at 25 MB in Outlook.com. Need to send a video demo or a large PDF? You’re forced to upload it to OneDrive and share a link instead (which supports files up to 2 GB). 

#4. No built-in analytics

Ever wonder if your email landed in a spam folder or why a client never replied? Outlook SMTP won’t tell you. Unlike services like Reply.io, it lacks delivery reports, open tracking, or reply rates. If an email fails, say, due to a blocked attachment, you’re left guessing.

SMTP alternative for cold outreach – API by Reply.io

SMTP is good for you if you have a small email list that you occasionally send newsletters to or run outreach campaigns to a few leads. But if you are about to scale your email operations, it’s best to look at APIs as an alternative.

Reply.io’s API for example, integrates LinkedIn outreach, SMS, and voice calls into one unified sequence. Imagine a prospect opening an email three times without replying. Reply.io’s API can trigger a tailored LinkedIn message or SMS follow-up, all within the same workflow. This cross-channel campaign is not possible with SMTP.

Sending emails via API will take advantage of Reply.io’s inbox rotation feature. This will cycle emails through multiple sender accounts to preserve domain reputation. So if one account nears its daily limit, the API rotates between multiple mailboxes to ensure smooth delivery.

In fact, emails sent through an API have a higher open rate of 42.4% compared to emails through SMTP which have an open rate of 30.5%. 

Source: 2024 Cold Email Deliverability Playbook

And to top it all off, APIs automate critical authentication steps (SPF, DKIM, DMARC), which are essential for inbox placement. For example, setting up these protocols via SMTP requires manual configuration. If you skip a step, spam filters might flag your emails as spam. Reply.io’s API handles this behind the scenes and ensures compliance without getting you involved.