Search the Logfile
Mailing stats are logged for your account. Typical log entries look like this:
2011-10-24 22:26 Mailing started: 3786 bytes to 2 reviewers = .007572 MB 2011-10-24 22:34 Mailing started: 3953 bytes to 2 subs = .007906 MB
Search the log to see various actions; for example:
To see mailings in Oct 2011, search for 2011-10.*subs
The search is not case-sensitive. Results are in reverse-chronological order.