Whether you’re setting up a mail flow for Exchange, an app server, or an IDP like miniOrange, Mimecast wants things done its way, and for good reason.
Let’s walk through how it works, what you need to know, and why even a “successful” connection doesn’t always mean smooth delivery.
What Mimecast Does in SMTP Relay Mode
Mimecast acts as a secure middleman. Instead of sending emails straight from your app, Exchange server, or hosted solution, you route them through Mimecast’s cloud. It reviews the message, applies any outbound security policies, logs the activity, and then relays it to the destination mail server.
That’s helpful for a few reasons:
- You reduce the risk of spoofing or unauthorised relay
- You centralise outbound delivery for compliance and tracking
- You get logging and auditing features that native SMTP might not have.
The tradeoff is that Mimecast doesn’t just accept any mail. It needs to trust the sender, verify the connection, and enforce strict authentication requirements. That’s where many setups go off track.
To make sure your emails are making it through, run them through InboxAlly’s free email tester to see how your setup holds up before deliverability becomes a guessing game.
Where Most Setups Go Wrong
Mimecast requires more than just an SMTP hostname and a login. The full checklist is longer than people expect.
You’ll need to:
- Create a dedicated user account inside your Mimecast dashboard. Don’t reuse an admin account. If that password leaks, it’s a security hole waiting to happen.
- Enable SMTP Email Submission and the Password Never Expires option for that user. Otherwise, authentication will fail.
- Use a cloud password, not an Active Directory one. This is a common point of confusion.
- Route mail through smart hosts. These vary by region—us-smtp-outbound-1.mimecast.com, for example, for the US. Always add both primary and secondary.
- Use port 587 with TLS 1.2 or higher. Anything below 1.2 is rejected.
If you’re using Exchange, you’ll need to edit your Send Connector, apply basic authentication using that new account, and make sure no other connectors are interfering. It’s a good idea to double-check, too, because many Exchange environments still have legacy connectors active in the background.
Why “Verification Successful” Doesn’t Mean You’re Safe
You’ve tested the connection, clicked the “Save” button, and it shows “verification successful.” That’s great, but it doesn’t mean your emails will actually arrive.
Since at this stage you usually won’t get a bounce or a notification, you’ll want to check Mimecast’s Message Center under Accepted Messages. If the email is there, it means it passed into Mimecast and was processed. If it’s not, something went wrong upstream, so you should check credentials, firewall rules, or port settings.
Why Inbox Placement Still Matters (and How to Fix It)
This is the biggest misconception: just because your mail is relayed through a secure, trusted service doesn’t mean it lands in the inbox. Mimecast isn’t responsible for your open rates, reply rates, or sender reputation.
Even a perfectly configured SMTP relay setup won’t stop:
- New domains from being flagged as cold
- Poor engagement from hurting your deliverability
- Spam filters from triggering if your content or sender reputation is weak
To fix that, you can try InboxAlly‘s email deliverability service. It works by simulating real, human-like engagement—opens, replies, moving messages out of spam—to build trust with ISPs and inbox providers. That trust is what gets your emails seen.
The Bottom Line
Mimecast SMTP relay is strict by design, and you want it that way. It protects your mail flow and helps with outbound policy control.
Once your setup is dialled in, you can send with confidence knowing your emails are passing through a trusted, policy-enforced gateway that’s built to keep threats out and compliance intact.
Want to make sure those perfectly routed emails land? Start improving your inbox placement with InboxAlly and get optimal results for your next campaign.