Having solved the problem of receiving attachments from Nextcloud mail, I ran into a new problem.
The problem is most likely due to the translation of the folder names (sent, trash, etc.).
Incoming messages are displayed normally, there is a problem with sent messages.
Sent messages are not displayed, but if you start sending letters, a new Sent folder (with an English name) will be created and newly sent letters will be added there.
As a result, I have two folders sent in English and in Russian. And most importantly, Nextcoud does not display letters from the folder sent in Russian.
By the way, the other Trash, Spam folders in Russian open and letters are displayed, there may be a difference in the length of the name or something else …
Sent by other mail services are also not displayed.
Tell me where to move, please.