So, people using Virtualmin or even Virtualmin people…
So, one can configure Virtualmin to auto handle mail coming in from other sources when Virtualmin server dies via backup email server functionality.
BUT, if users want to SEND mail from their domain using their email client, or even Usermin I suppose, this doesn’t help at all of course.
Does anyone have a nifty way to handle that?
This one is a non-issue. Any mail server to which you have permission to send through can send with the "from" field set to any address. Your ISP probably has one, Google has one (if you use GMail and have POP/IMAP access, I believe they also give you SMTP outgoing capabilities). However, unless you update your SPF record to indicate any or all of these servers are allowed to send mail on behalf of your domain, they may look spammy.
Really, the backup hold and forward mail server is a stop gap. If you want real reliability, you need a hot spare server that has all the same accounts. See the various threads on clustering and such for more on that topic (but beware there is no easy answer to the problem).