Office365 SMTP Settings: Step-By-Step Guide
Can’t send emails with Office 365? Fix your SMTP.
If your tool or app isn’t sending, it’s not broken — your SMTP setup is.
SMTP (Simple Mail Transfer Protocol) is what actually sends your emails.
If it’s misconfigured, nothing goes out. Or worse — it goes to spam.
This guide cuts through the fluff and gives you exactly what you need:
- ✅ Office 365 SMTP settings that work
- ✅ Which server and port to use (TLS vs SSL)
- ✅ How to set up SMTP relay the right way
- ✅ Fixes for the most common errors
By the end, your emails send clean, land right, and stop getting blocked.
Let’s fix it.
What Are Office 365 SMTP Settings?
SMTP (Simple Mail Transfer Protocol) is the technology that allows you to send emails from third-party tools through your email account.
For Office 365, it’s a key part of configuring your mailbox to send emails via applications like CRMs, email marketing platforms, or even custom integrations.
In simple terms, Office 365 SMTP settings are the technical details (server, port, authentication) that tell your app how to send emails using your Microsoft 365 mailbox.
These settings include:
- SMTP server address: The server that connects the email tool to your Office 365 mailbox
- Port number: The specific channel your email tool uses to communicate with Office 365
- Authentication: Verifying the tool's connection to your mailbox with credentials
And, this is where you can find these settings:
- Log in to the Microsoft 365 Admin Center
- Navigate to Mail > Setup > Domains
- Find SMTP settings under Mail Flow
Once you have these settings, you can configure any email tool to send emails through Office 365 securely and reliably.
Private vs Shared IPs: Impact on Deliverability
What Is the Office 365 SMTP Server Address and Port?
To configure your email tool properly, you need the right SMTP server address and port number for Office 365.
Here’s the info you’ll need:
And, here are some key points to remember:
- SMTP Server Address: The URL smtp.office365.com is what your app will use to connect to Office 365 and send emails.
- Port 587 (TLS): Port 587 is the recommended port for SMTP with Office 365. It’s secure and uses TLS encryption.
- Backup Port 25: Port 25 is less common and not always recommended for secure connections. It can be used, but may face blocking by some ISPs.
Make sure to use Port 587 with TLS for secure and reliable email delivery.
How to Set Up Office 365 SMTP Step-by-Step
🕒 Setup Time: ~5 minutes
Step 1: Verify and Enable SMTP AUTH
Before anything else, make sure that SMTP Authentication (SMTP AUTH) is enabled on your account.
This is required for securely sending emails through Office 365.
- Sign in to the Office 365 Admin Center
- Navigate to the Exchange Admin Center
- Ensure SMTP AUTH is enabled for your account
- Save the changes
Step 2: Insert Office 365 SMTP Settings
Now, insert the correct SMTP server settings into your email tool or app:
These settings will ensure secure and reliable email delivery. Use your full Office 365 email address and password for authentication.
If Multi-Factor Authentication (MFA) is enabled, you will need to use an App Password.
Step 3: Test Email
Once you’ve configured the settings, send a test email to yourself or a colleague. If everything is set up correctly, your email should be delivered without issues.
This step-by-step guide will ensure your Office 365 SMTP settings are correctly configured, eliminating common email delivery problems.
How SMTP Authentication Works with Office 365 (And How to Avoid Login Errors)
SMTP Authentication is the process your email tool uses to connect securely to your Office 365 account and send emails.
Without this, your tool can’t send emails through your Microsoft 365 mailbox.
But, does it mean your email really requires SMTP authentication for Office 365?
Yes.
If you want to send emails using Office 365, SMTP Authentication is mandatory. It verifies that your email tool has permission to send emails on your behalf.
Now, along with SMTP authentication, here are the typical login issues users face with Office 365 SMTP:
- Error Code: 535 5.7.3 Authentication Unsuccessful
Fix: Double-check your username and password. If MFA (Multi-Factor Authentication) is enabled, use an App Password.
- Error Code: 535 5.7.139 Authentication failed
Fix: Ensure SMTP AUTH is enabled in the Exchange Admin Center for your account.
Pro Tip: If you have Multi-Factor Authentication (MFA) enabled for your Microsoft 365 account, make sure to create an App Password. This is required for apps that can’t handle MFA directly.
By configuring SMTP Authentication correctly, you’ll prevent errors and ensure your emails are sent without interruption.
Cold Email Compliance Checklist 2025
SMTP Relay in Office 365: How It Works and When to Use It
SMTP Relay allows apps, printers, and internal tools to send emails through your Office 365 domain, without logging into an Outlook account.
Instead of each device using its own SMTP server, everything routes through Office 365 for secure, authenticated delivery.
🔁 Why Use SMTP Relay?
- Send emails from multiple devices or apps using a single domain
- Avoid setting up individual email accounts for each system
- Ensure authenticated, spam-free delivery
- Manage large volumes of email from CRMs, monitoring systems, or scanners
- Keep control in one place: Office 365
⚙️ How SMTP Relay Works
SMTP relay sends emails from your devices or apps (like a printer or internal tool) through the Office 365 SMTP server.
Here’s how it works step-by-step:
- Your device or app generates the email.
- It sends that email to Office 365’s SMTP relay server.
- Office 365 authenticates the source (usually via IP address or TLS).
- Once verified, it delivers the email to the recipient’s inbox.
- The recipient sees it as coming from your domain, not a third-party tool.
This setup eliminates the need to configure SMTP on every device.
Everything routes through one centralized, secure system.
And, this is how the SMTP relay works:
📍 When to Use It
Use Office 365 SMTP relay when:
- ✅ Sending transactional/bulk emails from internal apps
- ✅ Devices like printers need to email scans
- ✅ You don’t want to store login credentials on devices
- ✅ You want the email to come from your domain, not a generic third-party sender
🛠️ How to Set Up SMTP Relay in Office 365
- Go to Exchange Admin Center
→ Admin > Exchange > Mail Flow > Connectors - Create a Connector
- Choose "From: Your organization's email server."
- To: "Office 365"
- Choose "From: Your organization's email server."
- Choose Authentication Type
- IP-based: Whitelist device IPs
- TLS: Encrypt emails between your app and Office 365
- IP-based: Whitelist device IPs
- Whitelist Sending IPs
- Add static IPs of devices/apps sending emails
- Add static IPs of devices/apps sending emails
- Enable Relay
- Turn on “Allow all mail from this server.”
🔐 Reminder: Set Up SPF, DKIM, and DMARC
To avoid spam issues, ensure:
- SPF record includes your relay IP
- DKIM is enabled for your domain
- DMARC policy is aligned
Done right, SMTP relay via Office 365 gives you reliable delivery, centralized control, and inbox trust, all without overcomplicating setup.
Here’s an example of a basic SMTP relay setup:
With these settings, you can configure multiple apps or devices to send emails using Office 365’s secure SMTP relay system.
Office 365 SMTP Not Working? Here’s How to Fix It Fast
If your emails aren’t sending or you’re receiving errors, there are common issues that can prevent SMTP from working correctly.
Most of the time, these are some issues that we caught while setting up SMTP:
- Emails not sending: You click "send," but nothing happens.
- Authentication error: Incorrect username or password.
- Port timeout: Connection fails when trying to send an email.
Here’s how you can troubleshoot these issues and improve your email deliveries:
- Use Port 587 + TLS
Make sure you're using Port 587 with TLS encryption for secure communication.
- Use an App Password if MFA Is Enabled
If you have Multi-Factor Authentication (MFA) on, generate and use an App Password instead of your regular credentials.
- Whitelist IP or Domain if Using SMTP Relay
If you’re using SMTP relay, ensure your IP address or domain is whitelisted in the Exchange Admin Center.
- Test with Telnet
To check if the SMTP server is reachable, run this Telnet command:
telnet smtp.office365.com 587
This confirms whether the connection is working properly or not.
By following these steps, you’ll quickly identify and fix common SMTP issues.
What Primeforge Does (So You Don’t Have To)
Primeforge is your behind-the-scenes email infrastructure layer.
It is the One-Click SMTP Setup for Office 365, so you can focus on sending.
No more digging through DNS, guessing SMTP settings, or Googling port errors.
🔧 What It Replaces
❌ Manual SMTP setup
❌ Port & relay errors
❌ Cold email deliverability issues
❌ Constant DNS tweaks
❌ Inconsistent inbox rotation
✅ What You Get with Primeforge
- Auto-setup SMTP with Office 365 (no manual config)
- Real-time inbox health monitoring across domains
- Safe inbox rotation for bulk sending
- Built-in warm-up + throttling to boost inbox placement
- One dashboard to manage everything
How to Set Up Office 365 SMTP Using Primeforge
Setting up SMTP with Primeforge takes just a few minutes. Here’s the simple process:
- Connect your Office 365 mailbox to Primeforge
- Select “Auto Setup SMTP.”
- Primeforge automatically configures your SMTP settings
- It publishes them to your email tools and apps
- Start sending emails with optimized deliverability
That’s it.
No need to deal with ports, encryption settings, or error messages. With Primeforge, SMTP setup is fast, reliable, and fully automated.
And, here’s what our users have said in real-time about using Primeforge:
"I’ve been using a couple of other email infra forges for over a year and have been pretty impressed with deliverability.
Now I added Google mailboxes and have been using them for months as one of the early adopters to spread the load of 10K+ emails/day over to more ESPs.
Stellar deliverability on Salesforge and spam tests always come back green! Excited to spin up some Outlook accounts soon also."
– Rahul Lakhaney, Former VP at Gartner, now CEO at Enrich.so and Maximise
Start using Primeforge, and see how it can give you the automation, visibility, and control you need to send cold outreach emails at scale, without any headache.
Email Warm-Up Checklist for New Domains
Final Checklist: Office 365 SMTP Settings At a Glance
Setting up Office 365 SMTP settings isn't too technical. It’s quite simple if you follow it correctly.
Here’s the final checklist to make sure your SMTP settings are successfully set up:
- ✅ Correct SMTP Server: smtp.office365.com
- ✅ Port 587 with TLS Encryption
- ✅ SMTP Authentication Enabled
- ✅ Use App Password if MFA is enabled
- ✅ Test Email sent successfully
- ✅ Whitelisted IP addresses for SMTP relay (if used)
But, if you don’t want to get into the technicalities, you can use Primeforge to handle everything for you.
Primeforge automates the entire process, setting up SMTP, managing email authentication, and ensuring your emails are delivered smoothly.
By following this checklist or letting Primeforge take care of it, you’ll be sending emails with confidence, knowing your SMTP settings are properly configured.