This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
Michael K. Wilson wrote:(1) How can the Mailer class be set up, so it can send both HTML and plain text messages?
Multipart emails, I think multipart/mixed is needed but you'd have to read up on that a bit more.
(2) How can the Mailer class be structured to send e-mail to multiple recipients or one user?
You're already using "message.setRecipients(javax.mail.Message.RecipientType.TO, ..."; with that you can set mulitple To addresses. By changing the recipient type you can set CC and BCC addresses as well. And if I recall correctly, there is also an addRecipient(s) method.
(3) Would both of these features / actions require writing two overloaded methods for sendMsg()?
Probably, yes. With so many possible properties for emails, I'd be surprised if you would need just 3, or even just 5, methods.