Username length issue with SMTP in Azure Communication Services

Question

Hi,

I'm testing "Email SMTP as a Service" that is in preview within "Azure Communication Services". I was so excited to hear about it because we've just done an Exchange migration to O365, and this feature will help mop up the remaining SMTP client apps without having to deploy any SMTP relay infrastructure.
It was very easy to set up and get working for 2 test domains.... BUT...

The instructions state that a Service Principal must be used as the user. And with this, the username must be in the following format:
<Azure Communication Services Resource name>|<Entra Application ID>|<Entra Tenant ID>

This makes for a super long username. Mine was 94 characters! It worked in one SMTP client software but others I've tested have character length limits such as 60 characters.

The 2 pipes (or dots), App ID and Tenant ID guids mean that the username has 74 characters before the ACS resource name prefix is added!

I thought it might work with a UPN as username, so I added a user to the IAM Role Assignment, but that didn't work, returning error code "535 5.7.3 Authentication unsuccessful".

Is there a workaround? Because without one, the use of this SMTP feature will be severely limited.

Thanks,

Jonny S

Answer 1

@Jonny Sharp Good news! The product group wants me to gather some information from you so they can look into whether or not it would be possible to address this limitation.

Can you please send me an email to azcommunity@microsoft.com with the subject as ATTN: Bryan. I will then reply with the information we need for the product group to begin their consideration.