The Mailbox Limit for Gems of War is 100 Mail.
Due to technical limitations the mailbox limit is 100
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.
Legendary Tasks
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.
Guild Gifts
Guild gifts do not merge into a single mail! So ensure you are collecting them as soon as you receive them.
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.