a SMTP 550 error is pretty generic. You would probably need to look at the mailserver logs to get the rest of the error.
550 MIME boundary length exceeded (see RFC 2046)
That is not an error that I have run into personally. I do not see a whole lot of information on it.
Here is RFC 2046.
https://datatracker.ietf.org/doc/html/rfc2046
The word boundary appears in it 79 times.
I would definitely recommend that you reach out to your host about this.