error "No MimeBodyPart content"

[AnthillSudoku]

Hi,
I am using BlitzMail 0.7.1 from F-Droid with the same email provider (gmx) since few years ago. No issue till few months ago.
Now sometimes I got the error above and usually emails are sent even if the error appears, SMTP setting (like server, port; authentication, etc) seems OK as set up on Thunderbird or K-9 Mail

The server returned the following error. Please check your SMTP settings.
IOexception while sending message
Cause: No MimeBodyPart content