What causes 'syntax' bounces?

[clarkac]

My client email emailed about 800 contacts, and I notice a high number of bounces - more than half - with reported error of 'syntax'.    I tried sending one of these contacts a simple test email, and it seems to have worked - the original mailout was actually a graphics-heavy newsletter.  I haven't found any definition of what causes a syntax rejection.  So, can someone help and tell me what causes emails to be bounced with 'syntax'?