Review for Send Later by PowerShot
Rated 5 out of 5 stars
Work like a charm.
The mail is put into draft until sent. When it is sent, it will then put the sent message as standard in sent mail map of your client.
It will not send exactly at the minutes, because it depend on how often you let your mail check for sent and received mails. This time settings you can manage in your settings in Thunderbird.
To create your own collections, you must have a Mozilla Add-ons account.