Most companies like to append signatures to emails sent by their users, whether it's just a legal disclaimer or something more sophisticated.
The best and only approach is to use 3rd party tool for this job.
MSH Disclaimer & Signatures it's a server side solution and it's appears to be one of the best.
|Need Zimbra disclaimer? Easy way to set server side solution.|
Application can add different types of disclaimers depending on email format. It supports Html and Plain Text messages. It skips digitally signed and encrypted emails due to theirs nature (cannot be modified).
Disclaimers can be appended based on predefined rules, matched against message sender, sender domain, recipients and email content or even a time range. Application can detect already inserted disclaimer and will prevent from adding second one.
Newest release of application improve national characters encoding and give users control over both way charset conversion.
Visit official milter website and try an application for free.