WISH:
It would be really nice to be able to have a little drop-down on the
'Send-Receive' button to pick which account to send/receive for.

PROBLEM:
If a server is down and I want to see my mail for another account, I have
to Tools -> Settings -> Disable the account, then Send/Receive, then I need
to remember to Tools -> Settings -> Enable the account again.

POSSIBLE SOLUTION:
This could work like the "New" button that has a default behaviour and if
you click on the arrow, shows up some options such as

[Send/Receive]
  "Send/Receive"
    => account1@example.com
    => account2@example.com
    => account3@example.com
  "Receive"
    => account1@example.com
    => account2@example.com
    => account3@example.com
  "Send"
    => account1@example.com
    => account2@example.com
    => account3@example.com