When you receive "500 Line too long" error message in an NDR, it's because the message has more than the allowed number of consecutive characters in one line (ie, no line feeds) and the SMTP server or firewall / antispam filter bounced the message.
The line limit in Exchange Server is 8000.
WatchGuard and Ironport are known to have a default of 1000 characters. Other antispam filters may have a similarly low limits.
To change it in WatchGuard, edit the SMTP properties (in WatchGuard) and set the maximum email line length higher. For the exact steps, or for other firewall/antivirus software, you'll need to review the documentation that came with your software.