I’ve noticed that Duplicati mail messages aren’t always coming through. I suspect that Duplicati implements its own bare SMTP send scenario and doesn’t handle transient failure (4xx reply, not 5xx reply). Is that correct? (I haven’t have the time yet to get a compile working on my Mac so I haven’t looked at the code yet). In that case, with a 4xx reply it is at least useful to implement a retry after minimum of 60 seconds wait so the messages can also be delivered to mail servers implementing greylisting. Officially, 4xx returns are not failures, they are temporary failures and according to SMTP you need to retry delivery before giving up.
Duplicati uses MailKit. MailKit has full SMTP support. Have you logged the errors when Duplicati attempts to send the email? An error message can help users diagnose your issue.
It turned out to be a blocking issue on the other machine.
All good, glad your problem is solved and thanks for using Duplicati.