The email address is a Proton account.
This problem persists. Sent emails, usually with attachments, stay and multiply in the Drafts folder
Try adding the following string in the Diagnostics / Advanced field as per @heraroma advised below and see if that makes any difference.
If that makes no difference and you have an IMAP Proton account, suggest next to go to “Menu / Accounts” and remove and readd your account as per @Gary post below as could be your account is corrupted in some way.