Hello,
1 week ago I've implemented a new Exchange 2013 at a company which used SBS 2011 before.
But now its impossible to send a mail to a recipient on Exchange Online.
Everytime we get a NDR with the following text:
Service unavailable; Client host [xx.xx.xxx.xxx] blocked using Blocklist 1; To request removal from this list please forward this
message to delist@messaging.microsoft.com
If I run a SPAM-Blacklist Check on mxtoolbox.com I can see that the IP is on none blacklist.
How can i determine why this IP is on the Exchange Online Blacklist? I'm sure that no spam was sent over this IP, the new infrastructure is 1 week old...
Could there be an other reason ?