Gmail automatically copies sent messages to Sent Mail, so that is the one to subscribe. Right-click the account in the Folder Pane, Subscribe..., Refresh, and subscribe Sent Mail (unsubscribe Sent). Also, in Account Settings, Copies & Folders, uncheck 'Place a copy in' to avoid duplicates in Sent Mail.

Gmail groups the special folders on the IMAP server (more precisely what Gmail calls system labels, which includes the special IMAP folders but also other Gmail labels) under a [Gmail] folder. Recent versions of Thunderbird hide that parent folder from the user in the folder pane, but you may still see the folder hierarchy on the IMAP server in other places.

In particular, if you go to the account Copies & Folders settings and select Other for sent messages, you'll see the special folders used by Gmail grouped under the [Gmail] folder there, and that's how you may know which is and select the folder that Gmail uses for sent messages.

This thread may contain the answer to my question but I am not entirely sure.

Basically, in Copies & Folders, for sent messages, there are two options:

• "Sent" Folder on: xxx@yyy.zzz (where any of the configured e-mail addresses can be used)

• Other: where the options are:

      'Sent on xxx@yyy.zzz' (which is the account being configured)
      Any folder on any of the configured e-mail addresses

Superficially it seems that the first option ("Sent" folder on ...) should be the same as the second (Other: Sent on ...)

But when I accidentally configured two different machines with each of the options, I ended up with two different Sent folders. Mail sent from one machine went to one, while mail sent from the other went to the second folder.

(It seems odd that there could be two folders with the same name?)

Unfortunately, the TB documentation on this is quite weak:

"Place a copy in: With this setting enabled, Thunderbird will make a copy of messages that you send, and place them in the folder you choose."

Well yes, but how are the two options different?

My understanding of IMAP is meagre, but....

It seems like IMAP has a name "Sent" which is where it saves sent mail. This may map to a folder with a name like "Sent" or "Sent Mail" or "Sent items" or..... This seems like it might be what the first option is.

I am not clear what the second option (Other) does. Does it save the message to a folder called "Sent" explicitly or does it use IMAP's idea of where sent messages go? And if the latter, why did I get two folders called Sent?

Aaarrgghh!