Hello jap,
This is expected behavior with POP3 accounts. POP3 is designed mainly for downloading messages from the server, not for syncing changes or folders between the app and the mail server. That’s why sent messages are not automatically stored in the Sent folder on the server.
With IMAP, messages and folders stay synchronized between the app and the server, so everything, including Sent mail, remains consistent across devices.
If you want your sent messages to appear in the Sent folder on the server, we recommend switching your account from POP3 to IMAP. This will allow full folder synchronization and prevent issues like this in the future.