When you have Loyalty vouchers already set at the maximum cap limit - 1000000, the reward should be automatically sent to the Mailbox instead of being terminated and lost!
This ensures that if you want to upgrade some weapon or module, you can still use your Mailbox with such loyalty contracts afterwards to get your reward.
The Mailbox will keep the reward for one day or 24 hours. If not used within 24 hours, the reward itself will be terminated.
When you have the cap limit active and if you spent 50000 vouchers (950000 vouchers left), if the reward itself is 58500 vouchers, same mailbox will reappear and keep the difference.
This applies to all three factions:
Empire - Legion, Wardens
Federation - Armada, Vanguard
Jericho - Raid, Techs
(All of the loyalty contracts should be moved to the Mailbox, when they are already at the cap limit of 1000000 loyalty vouchers.)