strange problem about the email encoding

but sometimes, some user will report me that they recieved clobber mail.
i checked these mail and found that there is Base64 encoding.

so i cannot locate the problem, the error of my Application? or JMail, or SMTP server. or email client?

please help me

thanks and best regards

[443 byte] By [sharpMoon] at [2007-12-28]