If you have Mailpress 5.0 and use the mailing list addon to send emails, and your wordpress configured timezone is not UTC, chances are that if you edit a mail, select a list in the To field, leave it as "send immediately" and click on "Save as draft", your mail will be sent just as it would have if you clicked "Send".
Seems to be related to some bug fix in WordPress 3.0 related to handling local time. Before WordPress 3.0, if you have a different timezone than UTC and send an mail from Mailpress, it would show as if you sent it some time in the past (or future ?). With WordPress 3.0 this no longer happens, but there's the nasty bug I described above.