Support for authentication on SMTP relays

[adamcooke]

There seems to be a fair amount of need for username & password authentication on SMTP relays. This is a fairly simple thing to add so we might as well do it.

[WS365-Sybren]

+1

[sujitph]

+1

[dvdsim711]

+1

[davidpiccinini]

+1

[stevebrainng]

Very well needed.

[LeftoversTodayAppAdmin]

+1 Im having issues using Azure Communication Service SMTP relay because it needs credentials. I would love to use Postal for my platform but without this feature Im dead in the water. Please prioritize!

smtp_relays: ['smtp://smtp.azurecomm.net:587?ssl_mode=STARTLS']

Temporary SMTP delivery error when sending to 20.221.43.114:587 (smtp.azurecomm.net)

530 5.7.57 Client not authenticated to send mail.

Time: 0.04s

Support Ref: A0NPJDRO