Rated 4 out of 5 stars

Works pefectly with gmail. The only issue, and it's one which seems to be confusing some users, is that certain other mail providers are not compatible with the method for scheduling a message in the drafts folder.

For example, gmail is compatible, gmx is not. In the latter case, messages are kept in the drafts folder.

When you check the drafts folder, the first column header in the message list is the Send Later schedule time. If it shows any date/time at all, everything is fine and the message surely will send as planned.

If on the contrary, the phrase "Encryption compatibility problem" is shown, then you are out of luck because your email provider can not handle this.

This review is for a previous version of the add-on (9.0.11). 

I'm not aware of this specific incompatibility and I'd like to try to get to the root of it. Can you email me at send-later-support@extended-thunder.org and I will work with you to try to figure out what's going on and see if it's fixable? Thanks!
This may be an issue if you're using PGP on an account and you have Send Later configured to encrypt drafts. There is a workaround for this. Email me and I'll walk you through it.