Mail synchronization may fail when using email clients (e.g. Microsoft Outlook) with Symantec Endpoint Protection Small Business Edition cloud (SEP SBE) Email Protection. Restarting the client computer may resolve the issue for a time, but errors eventually recur.
The operation timed out waiting for a response from the receiving (POP) server 0x8004210a
Note: The mail server may also fail to recognize user credentials.
Disabling SEP SBE Email Protection is a workaround.
This article will be updated as new information becomes available.