I’ve read the pinned post about invitation emails but I don’t think this is addressed by any of the five points. I am trying to invite an email address (a valid one) and am getting an error message stating that it is not valid.
I just tried again this morning and received the same error when cutting and pasting a different email address. On a lark I tried manually typing that same email address and the invite went through fine. Then I tried cutting and pasting the first email address I had tried when I opened this thread, and that went through fine too.
Can you send an email to firstname.lastname@example.org with the email address you are trying to invite?