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 Guilds completing multiple Tasks at once.
NOTE: Players are encouraged to collect their mail regularly to prevent mail overflows.
Will the mailbox limit increase again?
We can't currently increase the mail limit anymore. Instead, in update version 4.2.5 (released January 15th 2019) we changed the mail to consolidate Legendary Tasks into a single mail.
If the mailbox has at least 51 unread mail the rewards will begin merging into a single mail. At this point, the mail will not refer to the player who has completed the mail or how many Tasks have been completed. This means that smaller Guilds can still see the Guildmate who has completed the Task as they complete fewer 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.