sendBuffer method reuses mimeMsg field to send emails. Sending emails is done asynchronously in threads. This results into sending invalid messages, for example subject may come from a different message. As a quick and dirty fix the "sendBuffer" method can be declared as synchronized. A better approach would be to create an instance of MimeMessage for every email to be sent.

P.S. While scrutinizing this module I also noticed that "cb.asList();" line in the "append" method seems to be doing nothing and hence brings an unnecessary minor performance impact.