The Mailbox Limit for Gems of War is 100 Mail.
Most games with mailboxes can only hold a certain amount of mail. Due to technical limitations, most games have small inbox limits. In August 2017 the Gems of War team increased the mailbox limit to 100 to help players collect more mail, after some rewards were missed due to completing many Guild Tasks.
Will the mailbox limit increase again?
We can't currently increase the mail limit anymore. Instead, the developers plan to introduce some methods to help you if you're about to go over the limit. For example, the team may be able to combine some mail such as Legendary Tasks.
Reimbursement for missing mail
Unfortunately, if you were offline from the game and missed some mail which expired due to an overflowing inbox the support team can't reimburse you for this.