The total size of your mailing exceeds the recipient server's limits
The recipient's mail server does not accept email's with the used number of mass/multiple recipients. (Eg. Using 15 email address in the 'TO:' or 'CC:')
For which ever email address failed, send the email to that user/address separately.
SMTP Error 450 5.2.3 Message Size greater than allowed by Remote Host
SMTP Error 552 5.2.3 Data size exceeds maximum permitted
SMTP Error 552 5.2.3 Message exceeds maximum fixed size