Maintenance for the week of November 17:
• [COMPLETE] ESO Store and Account System for maintenance – November 17, 6:00AM EST (11:00 UTC) - 4:00PM EST (21:00 UTC)
• Xbox: NA and EU megaservers for maintenance – November 19, 4:00AM EST (9:00 UTC) - 12:00PM EST (17:00 UTC) https://forums.elderscrollsonline.com/en/discussion/685477
It’s “fixed” by relogging or moving to a different zone. For some reason, teleporting/wayshrining to another zone refreshes the mail system.
It’s been happening for months.
Edit: I missed where you said you relogged already. Weird that that didn’t fix it initially.