Sporadic email access
 

I have the email client on my DINC set up to interact with my ISP's (AT&T) email. My carrier is Verizon.
Some days, email works as it should. I can get and send emails without any problems.
Other days, without making any changes and with the phone in the exact same location, I get an almost immediate "Login failed!" dialog. The only thing different is the passage of time. I make no settings changes and the phone is sitting in the exact same location. Time of day doesn't seem to make much difference. Some days I can't access emails for a couple hours, then I "magically" have access. Other days I have access that will go away and won't return for an hour, several hours, or for days.

Any ideas, besides "it's the [expletive deleted] network!", why this is happening?