IMAP Error after IOS 18 Update

Just got off the phone with a tech from my ISP and he helped me fix it by having me delete the account that wasn't receiving email from my phone and then adding it back. Here are the details:

I hadn't received any emails on phone from my business email account in a week, though they came in fine to the desktop Apple Mail. Also my personal account with a different ISP worked fine. I use POP, not IMAP, but didn't see any reason that should have anything to do with it. After checking my settings, updating to iOS 18.2.1, turning the phone off and back on after the update, trying to use Cellular Data to access the Mail app instead of WiFi, and seeing no answers in these forums that worked, I called my ISP to see if they had any knowledge of this problem. The tech had never heard of this error, but walked me through the same steps as above (with the exception of shutting down the phone), then suggested we delete the Mail Account and then add it back. After checking to see that there were no messages saved there that I didn't have on my laptop already, I tried that. And immediately 80+ emails that were still on the server came in, along with a test email I just sent from my personal account. So that is the only thing that fixed it.

I realize this could be a problematic solution if you have a lot of Sent emails that are only saved on your phone and nowhere else.

Same problem on iPhone 12, iOS 18.2.1 except I use POP. I am getting a POSIX 96 error. I haven't been able to download email on one of my accounts since January 4th. The other account (Comcast) works. I do not have this problem on my desktop. I have checked all my settings. WiFi vs Cellular does not make a difference. Switching to airplane mode temporarily does not work either.