Quota exceeded / mailbox for user is full (suddenly seeing this everywhere)

A few weeks ago, I got a "mailbox full" warning. To address this, I had to go into my webmail server and clean out my folders and empty trash. 

I've been on Cox email since its inception (it was home.com back then), and have never had to do this before.

Since then, many of the emails I have sent to other cox.net addresses have bounced back with "Quota exceeded (mailbox for user is full) (in reply to end of DATA command)". Several of my friends have expressed frustration similar to my own, and are giving up on their Cox email accounts. 

Maybe the quotas have been there all along, but why this sudden surge of bounced emails? What changed?

Oddly, I'm not seeing anything significant on Google about this. Wondering if it's limited to my geographical area (Phoenix, AZ)?