Building a mail system into a single-threaded text editor seems a bit silly to me...
Why's that? The (not gnus) mail system I use on Emacs will hang the Emacs process for a few seconds sometimes when I hit send while the Emacs process talks to my ISP's mail server, but that is the only way the mail system's single-threadedness ever really bites.
When I check for new mail on my ancient Pentium III system, the Emacs process hangs for less than a second or 2 usually.
Those delays are nothing compared to the time I waste waiting for Firefox to do something.