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How Can I Set Up SMTP to Send Emails to Exchange Online?

September 15, 2025 6 answers
Asked by TechSavvyGamer92
I'm currently working with Exchange Online and we use Outlook Classic for our email communication. We have several devices on our network that need to send reports via email to our clients. I've set up an SMTP service on one of our Windows Server 2022 machines, using IIS 6. However, when I try to send a test email to myself through that SMTP server, the email shows up in the Queue folder. Interestingly, when I direct the network device to send an email to a Gmail account through that SMTP setup, it goes out (even if it lands in the junk folder), but I can't seem to get it to send to our Exchange Online service. Any ideas on what I'm missing?
email configuration Exchange Online IIS network devices smtp System Operations

Answers

HybridHero98
We have a local Exchange set up in a Hybrid configuration for this type of thing, but with the upcoming changes from Microsoft, we're considering alternatives.
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QuickFixGuy
Why not just direct your SMTP traffic straight to Exchange Online using direct send? It could simplify your setup.
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RelayRanger
We use IIS as an SMTP relay, and while it’s mostly functional, it does have its quirks, especially with TLS certs. I'm actually testing a migration to Postfix, and it's working really well so far. I could share my current configuration if you're interested.
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EmailWhisperer99
You might want to consider using a service like SMTP2GO, especially since you're sending emails to clients. Microsoft has a policy against relaying external emails through their service, and SMTP2GO allows for better reliability and features like DKIM signing.
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AdminExpert123
Make sure to check your outbound connectors configuration in Exchange Online. If they're not set up correctly, that could be why your emails aren't going through.
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SMTPGuru88
I set up an on-premises SMTP relay on my Windows Server 2022 for devices that can't send directly to Exchange Online. Just send your emails from the SMTP relay to Exchange Online using the correct MX record, which should look like something.mail.protection.outlook.com. Don't forget to create an inbound connector in Exchange Online and choose the right authentication method.

DirectSender42: This is the answer! I have a similar setup with my send connector going directly to Exchange Online.

MXTracker77: Where exactly do I add the MX record in IIS 6.0?

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