sending email failed due to the following error: invalid MIME content

I can’t send any emails with attachments.  Large, small,
.xlsx, .rar, tried quite a few options and the following error message pops up “Sending email failed due to the following error: Invalid MIME content”

I’m having the same issue. The email we were trying to send doesn’t even exist anymore, not in OUTBOX, DRAFT or anywhere, yet eM Client keeps trying to send it and keeps giving me the error. I’ve even tried logging into  my email through the internet and it doesn’t exist there either. 

How do I made it stop?!

The Outbox will be the Local Folders Outbox. If that is not visible, go to Menu > Tools > Settings > General, and select Show Local Folders.

Been there, done that. __It still doesn’t show. But I still keep getting the same error. 

Can you post a screenshot of the error?

I think I left eMClient closed long enough that it cleared itself. It’s no longer giving me the error.