I can't see how it would happen automatically, a 4xx code doesn't mean 'try another mx response'...  I can add some code to try and detect this situation though.  What platform are you on? 

 

    ChrisP.

 

host mail-c6.linkedin.com[2620:109:c006:104::215] said: 421 4.3.0 Come back on IPv4 as this email does not pass SPF or DKIM when sent over IPv6 (in reply to end of DATA command)

 

I have a couple of messages sitting here with the above excuse from LinkedIn as to why they won't accept email via IPv6(they are NDR's). What part of a normal email server will drop back to IPv4 in this case? And even if we do, we still don't use SPF or DKIM at all here.

 

Lyle Giese

LCR Computer Services, Inc.

 

 

 

Don't bother.  I was more wondering about what Surgemail does.  And I just got the answer. 

More so, I informing others about this error message.  I found it stuck in my Barracuda's outbound queue.  It was for an NDR going back to LinkedIn.  

And it's not backscatter either...  The Barracuda will accept inbound all email for domain XYZ when the mail server is unreachable.  I filter incoming email for several small business servers running SBS or Exchange.  When the destination server is reachable, the Barracuda rejects before accepting the email when the destination server says 'no such user', therefore no NDR.

Lyle