Now that I've been using Gnus for all my personal email, I've been using Message mode a lot. One thing I need to do very frequently is CC or BCC people or groups. For example, I run a few non-profits and frequently have to CC the organizers@ list for our group with any email representing the non-profit to keep the other directors informed. While Message mode is awesome because I can simply type in the names of new headers, I would find it slightly more convenient of the headers I commonly use were already there. And if there's one thing Emacs is about, it's making things slightly more convenient:

(setq message-default-mail-headers "Cc: \nBcc: \n")

I also run my own mail server for my extended family since I've gotten for us. This means that if anything goes wrong with the server, I need to hear about it. So I'll enable debugging on sending mail.

(setq smtpmail-debug-info t)