Connection rejected my MTA (sender mimecast)

[Migration User]

We have a strange issue with mails coming from mimecast hosts. The sender is getting "Recipient server unavailable or busy" and trying multiple times to deliver the message. SMG is always rejecting the message without further information. IP connection class of the sending ip seems ok (class 4). The message will get through if the ip is whitelisted - but that's no solution (as sending ips may change from this provider).

The issue is listed in the forum multiple times:

https://www.symantec.com/connect/forums/rejected-mta-and-mimecast
https://www.symantec.com/connect/forums/messaging-gateway-defering-connections-mail-domain-service-provider

But no solution...

Any help would be greatly appreciated.

Thanks

[TomVie]

Hi Jürgen,

If the ip is not whitelisted, does the message never get through?

If thats the case i would tcpdump one of the connections.

If its SMG turn on debug-logging and investigate to get to the conneciton dropping cause.

Regards

Thomas

[jonathan sellors]

Hi,

I'm getting the exact same problem. Since the middle of February I'm getting a lot of Mimecast hosts being "rejected" in this manner even though the reputation is OK.

 

[Migration User]

We have been analyzing a lot with Symantec (tcpdump, etc.). Seems to be a problem with TLS-communications with some of the Mimecast hosts (not all, just some of them). Ticket is still open and Symantec working on a solution.

[TomVie]

Hi,

Thanks for coming back with that info and please keep us updated

Thomas

[Karen Toribio]

Hi, 

We are having the same issue too. Is there a solution provided already?

Thank you,
Karen