I tried Minecraft Multiplayer again for the first time since about the end of 2017, i.e. On servers, but I only get the message "You are not verified with Mojang" and I can't access the server. So how can I verify myself at Mojang?
Apparently you've never played MultiPlayer before.
This means that you have not yet confirmed your verification email (which you received when you created the account).
Search for the email.