Bypass throttling

[ISODT]

Our smtp server had been facing weeks ago an issue after sending mails to recipients using outlook 365.

all the messages are being delayed and set to "pending " status as shown below:

delayed. Reason: 4.3.0 - Other mail system problem ('451', ['4.7.500 Server busy. Please try again later from [**.***.***.***]. (S77719) [DS3PEPF000099DC.namprd04.prod.outlook.com 

we have changed twice the IP, and made sure it had been whitelisted and with good reputation, yet the issue is not solve.

we are using the web filter iron Port c600v. kindly check and advise.

[Dustin Anderson]

Changing IPs was probably a bad idea. MS has a learning for email volume from a server and has a daily limit of 1K from it. We ran into this same thing when we added a 3rd ESA. If you have MS365, you have to put in a ticket and have the server added to the "Warming" list. This lets them know this is a new server and to ignore the 1K limit as it learns the volumes that are normal.