Hello Drew Tomlinson,
Welcome to the Microsoft Q&A and thank you for posting your questions here.
I understand that you are in need of assistance to make sure your SMTP relay with Azure Communication Services works effectively.
With your explanations, you're very well on the right track with setting up Azure Communication Services (ACS) for SMTP relay. In the below lists of actons, observe what yo've done and what you have not as follow:
The first thing is to Register your app in Entra ID via Azure portal by navigating to Azure Active Directory > Select App registrations and then new registration > Fill in the required fields and register the app > and once registered, go to the app’s Certificates & secrets section to create a new client secret. Note down the Client ID, Tenant ID, and the Client Secret.
Secondly, In the Azure portal, navigate to Azure Communication Services > Select your ACS resource and go to Access control (IAM) > Create a custom role with permissions to send emails. You can clone the Reader role and add the necessary actions for email sending.
Then, use the following SMTP settings:
SMTP server address: smtp.azurecomm.net
Port: 587
Username: <ACS Resource Name>.<Entra Application ID>.<Entra Tenant ID>
Password: The client secret you generated.
After all the above, you will need to test mail using any email client to test sending emails. Here I used a PowerShell for an example:
$Password = ConvertTo-SecureString -AsPlainText -Force -String '<Client Secret>'
$Cred = New-Object -TypeName PSCredential -ArgumentList '<ACS Resource Name>.<Entra Application ID>.<Entra Tenant ID>', $Password
Send-MailMessage -From '******@yourdomain.com' -To '******@gmail.com' -Subject 'Test Email' -Body 'This is a test email.' -SmtpServer 'smtp.azurecomm.net' -Port 587 -Credential $Cred -UseSsl
For more steps and detailed guide, you can refer to the Microsoft Learn links here: https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email-smtp/send-email-smtp
I hope this is helpful! Do not hesitate to let me know if you have any other questions.
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful.