Just a quick aide-memoire and a note to anyone else who’s caught out… when using Python’s smtplib module to send email. If you’re like me, you may have missed the following documented behaviour:

This method will return normally if the mail is accepted for at least one recipient. Otherwise it will throw an exception. That is, if this method does not throw an exception, then someone should get your mail. If this method does not throw an exception, it returns a dictionary, with one entry for each recipient that was refused. Each entry contains a tuple of the SMTP error code and the accompanying error message sent by the server.

In other words, sendmail will return successfully even if some of the recipients couldn’t receive the email. You can work out which recipients failed from the dictionary returned. I’d assumed that if *any* recipient didn’t get the email then an exception would be raised. As I say, the actual behaviour is clearly documented, but just in case…