A common issue in using ExQuilla is configuring the correct outgoing server. For the vast majority of ExQuilla users, outgoing emails should be sent through the Microsoft Exchange server that is connected to the ExQuilla account. But sometimes users try to setup the Microsoft Exchange server as an SMTP server, which usually fails.
The following instructions and screenshot are taken from the Thunderbird account manager, which is reached from Tools/Account Settings (in Windows and OSX) or from Edit/Account Settings (in Linux). These examples show a system with two configured accounts: an IMAP account (email@example.com) and an ExQuilla account (firstname.lastname@example.org)
The normal Thunderbird user interfaces allows you to setup Outgoing SMTP servers. The ExQuilla account should NOT be configured here, as it is not actually an "SMTP server". So typically you would not see the ExQuilla server listed in the global Outgoing Servers configuration screen, as shown here:
Note that the ExQuilla account "email@example.com" (which is a test account by the way, don't send email there!) does not appear in the list.
When you configure the outgoing server that will be used by the ExQuilla account, an additional server will be listed for the ExQuilla server. That is the correct server to use for the vast majority of users who intend to send messages through the Microsoft Exchange Server. The following screen shows the correct configuration for Outgoing Server for the ExQuilla account:
If you are having trouble sending messages using ExQuilla, an incorrect configuration here is the most common cause.