Most people check their mail upon seeing the little orange icon, as often its patch notes, some item in the mail, or something a friend/guild mate thought was important enough to mail you about. If there is a Mist Tank in your mail you accept it and can save it in your inventory indefinitely.
Quite frankly, if you haven't played for 30 days then normally you have quit, so it doesn't matter what is in your mail.
~Gwen
I think the deletion thing after 30 days mechanic should only apply to mail that's been read. Players generally collect attachments immediately after reading a message (if there is an attachment) though Mikuknight's suggestion should still be implemented also (or an automatic collection of the attachment after 30 days) just in case.
Sure, it wouldn't affect too many players in the long run, it would still POSITIVELY affect those it does affect.
I know that I've taken long breaks from MMO's I like because of Life In General (school, even once you're in college, is still a drag and a half), and to know that mails sent to me with attachments wouldn't be lost to time would be, to me, quite appreciated. After all, sometimes things force you to be away from a game you enjoy, and as we all know life marches on -- meaning that if you're forced to part with the MMO for a long while, if this idea's not in place then that recipe your friend sent you because he knew you'd dig it, or that accessory your guildie had a spare of that they knew you were saving up for, would be lost to deletion. So, with this in place, that would not happen, and instead you'd be picking the game back up with a pleasant surprise in your inbox, and you would be happily continuing your regular gameplay (with perhaps a thank-you sent to the parties involved) with no worries at all.
It might not have a major impact on the game as a whole, but it would still be one of those little things that makes an MMO a bit nicer to its patrons/players, y'know? (And if they would only remain un-deleted if the attachment was still attached rather than having already been accepted, then it wouldn't even NEGATIVELY affect anyone.)
FYI, you can recive the mist tank without using it. It sits in your inventory under the "useable". Also, I'm pretty sure if the mail contains some item, you automaticaly accept it after 30 days. If not, it probabaly just overrides the delete system already, like you suggested.