Email sending errors

Not sure if you can help me with this.  Running 4.3.10

When a user preforms certain activities like signing up for an even it automatically generates an email to them.
Occasionally I get the following email in the websites admin email inbox:
    host smtp.antispamcloud.com [38.109.53.37]
    SMTP error from remote mail server after end of data:
    550 Maximum line length exceeded (see RFC 5322 2.1.1).

As it doesn't show me the complete email it attempted to send so I can't troubleshoot it.  Can you help?