My work recently changed over to using Office365.com for their email. As long as Office365 supports IMAP and SMTP (it does) I don’t mind so much, as I can continue to use Thunderbird on my desktop to access my email.
However, because the mail servers are no longer located “on-campus” the lag to the SMTP server is much more noticeable than before. When I pressed the “Send” button, Thunderbird would sit there for 20 seconds with a progress bar sending the mail, interrupting my flow of email processing much more noticeably than when the server was hosted on the same network.
So I tweaked a few advanced settings in Thunderbird by going into Thunderbird Preferences (Edit->Preferences), clicking the “Advanced” tab, then selecting the “config editor” button and promising to be careful. [Newer versions of Thunderbird have these options under the tools -> Options menu. ]
Telling thunderbird to not show the progress indicator only hides the dialog, but the compose window still sits up on your screen (with no visible indicator of what it’s doing) until the mail is sent, so you also need to turn on “sendInBackground” to make the compose window disapear immediately after you press send.
This makes the message go immediately to your Local Folders -> Outbox, but DOES NOT SEND IT, until you select File -> Send Unsent Messages. Hence the “offline.send.unsent_messages 1” which tells Thunderbird to send unsent messages (from Folders -> Outbox) whenever it can. It still takes 20-30 seconds for the message to be sent, but it is done in the background now.