The Mailbox Limit for Gems of War is 100 Mail. 

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.

Legendary Tasks

If the mailbox has at least 51 unread mails, 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.

Mail consolidation

Guild Gifts and Mastery rewards are just some of the items that do not condense down into one item, like Legendary tasks.

Reimbursement for missing mail

If you were offline from the game and missed some mail that expired or was due to an overflowing inbox, the support team can't reimburse you for this.