We have successfully used an SMTP relay service for integrating on-premises applications and appliances to route mail through our Office 365 tenants. For years this has worked seamlessly, almost in a set-and-forget type of a configuration. Over the past several weeks, I have assisted a handful of clients in troubleshooting why their SMTP relays stopped working (after working reliably for years). In all instances, the issue was traced to the relay mailbox being full! After purging content from the mailbox, the relay would resume working again.
Mailboxes look like this:
Our SMTP relay accounts only use a kiosk license with 2GB of storage and while that isn’t much storage, we never had a need for a higher license. So what changed? On June 1st, Microsoft made a service change to the SMTP submission access. The two changes made were:
- items submitted by this protocol will be saved into the sent items, and
- the number of concurrent connections will be capped to 3.
For the most part, the behavior change to save all items routed through the relay into the sent items of the account is what consumed all the same. From the screenshot above, you can see that 50% of the storage is within the Sent Items folder.
- Setup a retention policy to remove items older than X days (the mailbox above has a 7 day policy).
- Stop using a kiosk license and switch to the Exchange Online Plan 1 license. The license change would give you and additional 48GB and is only $2 more. It would still be a good idea to setup a retention policy but could be for a longer retention (30 day).
While I can appreciate the change as it can make some troubleshooting efforts much easier, the change has certainly caused some confusion on the first few occurrences.
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