In game mail spam. Ugh. So annoying and probably difficult to tackle for the developers. However, I do have an idea!

First, all mail from ZOS staff such as GM responses or customer support should be marked with an official logo of some sort. Personally, unless it has items/gold attached to it, I delete all of my mail without reading it, even if it's from an official employee because, to me, they look no different than spam mail/phishing attempts. Just look at the mail gold spammers are sending us now that are labeled as "Raw Materials" and "Item Sold". Just a matter of time before we see mail titled as "Account Suspension" followed by a body of text asking us to reply with our password to confirm or some crap.

This is exactly why you want to mark your mail, ZOS! First, people will fall for phishing attempts if your mail looks no different than phisher mail. Also, people like me will just purge any and all non official looking mail, no matter how good your grammar and punctuation is, because it's as good as phishing attempts to me.
Now, if you're already marking your mail, then just ignore this. However, last time I received an expected response from ZOS via the mail (note, I was expecting it, hence why I didn't delete it immediately), it was not distinguishable from other mail.

Second, I think all player sent mail should also be distinguished in some way, Not entirely necessary, but it does help to distinguish it from expired auctions, sold items and hireling mail.

Now, this is the meat of it. I think that players should be given an option to block mail from other players with a few conditions. The first condition would be to block all mail from players except for players on the friend list. The second condition would be to block all mail from players not in your guilds. Of course, you can toggle both on and off individually. Either way, the point here is that a player can use their guild rosters/friend list as sort of a 'hey, I trust these people' list (because that's exactly what they are) and block mail from all other strangers.

I don't see this as anything but a benefit to the sanity of players that will help put another nail in the coffin of the ever ongoing gold seller spam.