Adding another character to the friends list will result in no longer getting the “this character might be someone you don’t know” warning & three-second cooldown when sending mail. It will also allow you to auto-populate the sender field by typing the first few characters of your friend’s name. But after logging out and logging back in the mail interface no longer respects the existing friend. The sender field will not auto-complete and you will get the “someone you don’t know” warning & 3 second cooldown before you can send.
I know this is the Classic forum but I have been getting it for months all the time on retail. For me, I mostly am mailing between my toons on my two accounts that are linked by TSM so I believe it is a TSM issue.
In fact, in thinking more about it, I found discordapp com/channels/244841024252280832/244841469070802944/976170621694251038 in the WoW Economy/TSM discord
I just tested it and, if I remove the toon from my friends list, I can send the mail no problem. In my case, when I removed the friend and closed the Friends List window with the X, TSM immediately re-added them but, if I removed them and immediately opened the mailbox, they were not re-added but Classic might work different.
When I google’d
“this character might be someone you don’t know” site:us.forums.blizzard com <
this post was the only one that showed up and there are only 9 posts going back to at least 2017 in that discord so I don’t know if it is super common but it certainly has wasted a bunch of my time