In a D7 (up to date) site where the User Import module had been working successfully, it is now generating an error for email-sends and failing to send the Welcome Email that should result from importing a .csv list of users. Tried several email addresses as a test, they did not send. It was working for several years but a lot of things have been updated since then – core and numerous modules. I don't see any details in the site log that clearly define the nature of the error. [The error message in Log is just "Error sending e-mail (from email@example.com to name-of-recipient.com)."]
Using the "send test email" option of the SMTP Authentication Support module seems to verify there's nothing wrong with that. I tried going back several versions for both the SMTP Authentication Support and User Import modules but nothing improved. What are some other things to check which could be related to the failure?
Update: When one stumbles upon a solution, of course it's a relief to have the problem solved but it's dissatisfying when it's not understood exactly what happened. I did not have the Mailsystem module installed, only SMTP Authentication Support and as mentioned, that had been working. I installed Mailsystem to see if maybe that would work better. It did not, but... if I have both SMTP Authentication AND Mailsystem installed, with Mailsystem's config set to "SmtpMailSystem" as the default Delivery method – then it works. Glad it works of course. But the problem is, I don't understand what's really going on since only one module was previously required to make it work.