What is the difference between the free trial version and the full version of Mac Mass Mailer?

There is absolutely no difference in functionality.

The trial version is exactly the same as the full version you get once you purchase Mac Mass Mailer.

There are a few limitations though:

• The trial version expires in 30 days
• You can send not more than 100 messages at a time (but if you hit the Send button again, you will be able to send 100 more messages).