Dear Polaris.
I has corrected guild coin ,surge reword , contest reword , deal purchase from 2nd Aug. on my mail box.
It was deleted today without any notice.
(Mail box deleted from 2nd Aug. to 11th Sep.)
Please check this issues and feed-back to me.
Our guild members want your answer also.
The mailbox messages were not lost. There was a change in 1.12.2 that mailbox messages would auto-collect if they were more the 30 days old. Players were hoarding mailbox messages and it was causing performance issues. I apologize that there was no announcement about this.
I have reviewed the logs for a couple players who reported this, and the tokens were indeed applied to accounts correctly, and nothing is missing.
But what would happen to guild coins if our bank is full? Where will those coins go to? Disappear into thin air? OMG, I sure hope not 
If the Guild Coins are auto-collected, it will push you over the limit. They won’t disappear.