You can specify the location for email storage, regardless of any Thunderbird defaults.

This should be found under Edit > Account Settings > <account name> > Copies & Folders.

Those settings already say "Place a copy in 'Sent' Folder on: [...]@fastmail.fm," but as I said, TB isn't finding that folder, it's creating its own.

right click the account in the folder tree. Select subscribe. Ensure the sent folder for fastmail is subscribed. Now go back to the setting that Fabian gave and chack again for the correct folder.

The issue you describe is almost always a matter of linguistics. The server calls it "sent mail" and Thunderbird calls it "sent", or in the case of localized Thunderbird version the server may be configured in a different language to Thunderbird. However once the folder is subscribe and manually corrected all should be good.

FANTASTIC. I'm new to Thunderbird, so I had no idea that dialog existed. Thank you!

I am glad to hear that your problem has been resolved. Thank you for selecting the answer that solved the problem, this will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support!

I have partially solved my problem with the answers.

What remains though is that TB calls my sent folder "Sent" and keeps the messages sent from TB in this folder whereas my provider calls is "Gesendet" and keeps online sent messages (messages sent via login on the providers internet page) in "Gesendet". Therefore, TB and my provider both have these two folders and the messages are kept separately, depending on how I sent it.

I would like to change the "location" mapped to the sent folder in its Properties / General Information / @imap.gmx.net/Sent to Properties / General Information / @imap.gmx.net/Gesendet

so that all messages will be kept in TB in the folder "Sent" and all messages will be kept on by my provider in its "Gesendet" folder.

I just cannot change the wording in the respective line of the location.

What can I do to solve this? Thanx for any help Holger

Right click the account in the folder tree and select settings. open the copies and folders for the account and change the location for "place a copy in" from "Sent folder on" to other and select the Gesendet from the list.

As until now the two have been separated and apparently not synchronizing with the server, it might be an idea to backup the local Sent folder under "Local Folders" as it may simply disappear when you select the alternate location.

Complete the process by dragging "Sent" mail to "Gesendet" so there is one folder with one set of mails going forward.

Thank you, Matt, for your reply and solution.

May I dare to ask if there is a way to leave the messages sent from TB in the "Sent" folder and map them to the providers "Gesendet" folder? (Now, I have a mixture between two languages in TB ,and I would like to stay in English, if possible).

Thank you again, I hope this question is not too ideosyncratic, Holger