The email body has been created with a maximum size of 255 characters, this is not enough to send any decent email to a client. 

I would suggest it be handled in the same way we planned on handling

https://openvpms.atlassian.net/browse/OVPMS-1120

ie increase the field to a 5000 character limit.  My understanding is that these messages are not saved so going even higher should not influence the database size or speed.

However I have noticed that this system is not based on an archetype so I assume its hardcoded into the web app