I'm unaware of any big-time ISPs that don't have email problems from
time-to-time.  I include my institutional mail system in that statement.

Hackers, spamers, spoofers don't help.

These mail and embedded systems do not run themselves and take constant
attention.

routers
domain name system
circuits
mailing list manager
mailers
smtp client
smtp servers
authentication systems
anti-spam filters
upgraded software

all get in the way of reliable delivery from time-to-time.

/Pete