Regional FlagMail lasting longer than 30 days?Source
Target Source
#1 - 2018/06/17 05:49:00 PM
If you send mail to a character who's subscription is not currently active, will the mail get deleted from their mailbox after 30 days, or will it stay until they reactivate their subscription, and they will still receive it?

Forum Engagement
Target Source
#5 - 2018/06/18 10:47:00 AM
17/06/2018 21:05Posted by Gené
To my knowledge thats how it is too, if you send a mail to a character and its not picked up for 30 days, you will have it sent back to the character it was sent from, and if that one isnt looted for another 30 days then it gets deleted with its content.

Correct, this is how it works!

There are a few exceptions (messages sent by the system or us Game Masters may last longer than that, mails from the Auction House are purged after thirty days as there is no sender to return them to) but your summary is accurate, on a general level :)

Due to the temporary nature of the inbox system I would caution against using it as storage and advise to use a proper tool instead (e.g. inventory space or the Reagent Bank).