This is what MS Outlook does, as much as you might hate the program. Outlook gives you lots of information on whether and when your mail is still sending, if there was an error, etc. It is tolerant of closing the window or losing network connection. And sending a mail doesn't get in the way of doing other things in the program.
maybe i don't have experience with many other mail programs but this is what thunderbird does also, its not unique to outlook. I hate outlook because its slow (UI search and UIwise) and because its stupid enough to think that storing all its mail data in a monolithic file or small number of files is a good idea, hello corruption problems.