Hello,
It looks like when you attempt to mail someone who has not logged in since a restart it will not work. Performing /seen and then mailing the person will allow you to though.

Please define "will not work". There are many many many ways something can not work. The details that involve you knowing how something doesn't work needs to be shared including any technical information that can help such as your server version (type /version) and your EssentialsX version (type /ess reload)

Essentials: 2.0.1-b411
Paper 812

The problem is when you mail someone it " it will not work"
Meaning you cannot mail someone, the error is the player cannot be found.
If you /seen the person and then proceed to mail them it seems to allow you to, is this because the player is not in some sort of cache?

Closing due to inactivity. You can simply run /seen or any other command that would load the user's data before sending the mail. If there is still a problem, please open a new issue.

/seen does not work for me. It also gives me the "Player not found" error