Bugzilla

DevTeam,
Have you tried the latest beta to see if it fixes the issue?

I'm assuming the your steps below are with the release and not the beta?

> I go to the calendar plugin, and add an event.

I assume it doesn't try to send a mail at this point.
Also, was there a message send by TB recently (within your server's 30s timeout) before this point? If so, the connection to SMTP server may still be open. 

>I invite someone else to this event.
I click to save the new event.
The plugin attempts to send a calendar invitation email to the person I invited.
It throws an error message, the invitation is never sent (because the 'cached' connection timed out).

I don't know why the message would not be sent. I thought the timeout was due to your server timing out the connection 30s after the message was sent because, with the current release, there is no QUIT sent and the connection is just kept open until TB is shutdown or the server (or routers) times it out.
Anyhow, with the beta (and the future release) a QUIT will be sent 5s after a message is sent unless another message is sent within the 5 seconds.

Maybe if you could repeat the step of comment 28 with your release version and  with with smtp.loglevel at ALL and with timestamps also enabled, it might show what is going on. Then maybe repeat the process with the beta and you should see better behavior and also a QUIT sent 5 seconds after the message is sent.

DevTeam,
Have you tried the latest beta to see if it fixes the issue?

I'm assuming that your steps below are with the release and not the beta?

> I go to the calendar plugin, and add an event.

I assume it doesn't try to send a mail at this point.
Also, was there a message send by TB recently (within your server's 30s timeout) before this point? If so, the connection to SMTP server may still be open. 

>I invite someone else to this event.
I click to save the new event.
The plugin attempts to send a calendar invitation email to the person I invited.
It throws an error message, the invitation is never sent (because the 'cached' connection timed out).

I don't know why the message would not be sent. I thought the timeout was due to your server timing out the connection 30s after the message was sent because, with the current release, there is no QUIT sent and the connection is just kept open until TB is shutdown or the server (or routers) times it out.
Anyhow, with the beta (and the future release) a QUIT will be sent 5s after a message is sent unless another message is sent within the 5 seconds.

Maybe if you could repeat the step of comment 28 with your release version and  with with smtp.loglevel at ALL and with timestamps also enabled, it might show what is going on. Then maybe repeat the process with the beta and you should see better behavior and also a QUIT sent 5 seconds after the message is sent.