Use hostname or mail.theirdomain name for email?

Using SSL authentication for sending emails of course, shared customers get SSL warning if they use as their SMTP server. Hassle.

So, they can of course use the hostname of the server since it has a valid SSL certificate.

For bandwidth purposes, I presume this means they are not using their own allocated bandwidth to send email though. Is that true, or not?

How do people get around the problem? Do they have their users deal with the SSL certificate mismatch, or something else?

Yup, I just direct folks to use a centralized domain name I setup with SSL support.

It never amounts to all that much bandwidth – I let folks use it as much as they need to, and just write it off as a cost of doing business.