Once you reach 3,000 or more Stamina on your account, it is no longer possible to claim additional Stamina from the mail. This extends to any mail that includes Stamina, so the limitation may prevent you from obtaining other rewards as well.

This is an intended limitation, and the way around it is to consume Stamina until you're below the 3,000 Stamina limit. The quickest way to do that is by using the Sweep feature.

Sweep

In Heavenhold there is a building called the Guardian Base Camp. As you progress through Guardian Tales' storyline, you will unlock this facility and gain access to Guardian Levels.  Once you reach Guardian Level 26, obtained after clearing World 3, you will unlock the ability to Sweep Resource Dungeons.

What is Sweep?

Sweep is a feature that allows you to instantly obtain the rewards from a dungeon without having to manually or auto-battle through it. In this case, it will allow you to consume all 3000+ stamina that is hindering your ability to claim more mail in under 1 minute. If you encounter issues using the Sweep function, please see the following article HERE.