I find I mail alts almost 99% of the time. Unfortunately, TSM Mailing doesn't make it quick and seamless to mail them:
1) Open Mailbox
2) Click Contacts
3) Click Alts (or Recently Mailed)
4) Click Alt's name
5) Right click items
6) Hit send
7) Repeat from step 1, even if you need to send more mail to the same alt
What I suggest is that the Recently Mailed or Alts contacts be surfaced on the main mail screen so you can just click their name on the Send pane, skipping steps 2 and 3.
Chad Cloman commented
The TSM UI seems to go by the philosophy of, "why do something in one click when you can do it in two or three instead?" This is one of the more obvious examples, and I would love to see it changed.