A 503 – Server Unavailable error from Exchange Online is one of the most common failures during EWS migrations, and it always means: Exchange/Office 365 is temporarily refusing your connection because of throttling, overload, or mailbox database movement.

This is NOT a permanent failure. It is a temporary block.

Causes:

1. Throttling (Most Common)

Microsoft heavily throttles EWS:

•     Too many EWS calls in a short time

•     Multiple Bind, Load, or FindItems too fast

•     Large messages or calendar items

•     Public folder access

•     Long-running requests

•     Too many concurrent connections

Exchange Online stops responding and returns 503. You have already seen:

•     “Too many concurrent connections opened”

•     “Mailbox database temporarily unavailable”

•     “Underlying connection was closed”

This is all the SAME throttling family.

2. Network-level cutoff

If Exchange stops responding briefly, EWS throws a 503.

Resolution:

Wait and Retry: Wait a few minutes before trying again.