Rated 2 out of 5 stars

This is a very useful add-on, in principle. But, I had to stop using it because of its incompatibility with Enigmail. Apparently this is a known bug, and I'm waiting for a fix. If it was fixed, I would give it five stars.

This review is for a previous version of the add-on (5.0.1). 

The incompatibility with Enigmail is not a bug in Send Later, it's a bug in Thunderbird, or more accurately, a limitation in Thunderbird's add-on integration framework. It doesn't seem fair to give Send Later a bad review because Thunderbird's framework isn't functional enough to allow Enigmail and Send Later to play nicely together without breaking the rules.

Having said that, the maintainers of Enigmail and I have worked together to find a way to "break the rules" collaboratively, and as a result, the next release of Send Later -- due out this week -- will be fully compatible with Enigmail.