Socket connection closed: receiving mail to Zoho fails

Socket connection closed: receiving mail to Zoho fails

Hi

We have a Zoho account in place and our mx records are all updated to point to the mx and mx2 subdomains of zoho.com. The majority of mail inbound is fine, however we've had a few reports of failures that appear to be related to the mx and mx2 servers terminating connection with sending servers prematurely.

Here's one example (obfuscated to hide email addresses):
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=  Session Transcript  =
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  Session 219174; child 0001
  Parsing message <xxxxxxxxxxxxxxxxxx\pd50001086283.msg>
  *  From: sender@inbound.co.uk
  *  To: recipient@ourdomain.co.uk
  *  Subject: RE: Quote
  *  Size (bytes): 49872
  *  Message-ID: <016901d1c196$1a2f2ed0$4e8d8c70$@bizspace.co.uk>
  Resolving MX record for ourdomain.co.uk (DNS Server: 223.254.100.3)...
  *  P=001 S=000 D=ourdomain.co.uk TTL=(0) MX=[mx.zoho.com]
  *  P=005 S=001 D=ourdomain.co.uk TTL=(0) MX=[mx2.zoho.com]
  Attempting SMTP connection to mx.zoho.com
  Resolving AAAA record for mx.zoho.com (DNS Server: 223.254.100.3)...
  *  mx.zoho.com added to internal AAAA lookup black-list
  *  DNS server reports no valid records for the requested type found
  Resolving A record for mx.zoho.com (DNS Server: 223.254.100.3)...
  *  D=mx.zoho.com TTL=(4) A=[74.201.155.125]
  Attempting SMTP connection to 74.201.155.125:25
  Waiting for socket connection...
  *  Connection established 223.254.100.21:64464 --> 74.201.155.125:25
  Waiting for protocol to start...
  Socket connection closed by the other side (how rude!)
  *  Socket error 10053 - Connection abort.
  Connection closed
  Attempting SMTP connection to mx2.zoho.com
  Resolving AAAA record for mx2.zoho.com (DNS Server: 223.254.100.3)...
  *  mx2.zoho.com added to internal AAAA lookup black-list
  *  DNS server reports no valid records for the requested type found
  Resolving A record for mx2.zoho.com (DNS Server: 223.254.100.3)...
  *  D=mx2.zoho.com TTL=(4) A=[74.201.155.220]
  Attempting SMTP connection to 74.201.155.220:25
  Waiting for socket connection...
  *  Connection established 223.254.100.21:64465 --> 74.201.155.220:25
  Waiting for protocol to start...
  Socket connection closed by the other side (how rude!)
  *  Socket error 10053 - Connection abort.
  Connection closed
  Attempting to send message to smart host
  Attempting SMTP connection to relay.thirdparty.net
  *  relay.thirdparty.net found in internal AAAA lookup black-list
  Resolving A record for relay.thirdparty.net (DNS Server: 223.254.100.3)...
  *  D=relay.thirdparty.net TTL=(1) A=[80.168.45.10]
  Attempting SMTP connection to 80.168.45.10:25
  Waiting for socket connection...
  *  Connection established 223.254.100.21:64466 --> 80.168.45.10:25
  Waiting for protocol to start...
  <-- 550 No SMTP service for unauthorized users
  --> QUIT

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=    End Transcript    =
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So it seems that the sending server tries the mx.zoho.com address first, which times terminates the connection (how rude!), and then attempts the second mx2.zoho.com address, which also terminates the connection. Finally the sending mailserver attempts to relay via a another smtp relay server without authentication, which returns a 550 error.

I'm trying to understand where the fault lies - either with us/zoho or with the sending party. Could some assist in troubleshooting this please - I'm happy to run any diagnosis required on DNS / smtp etc.

Thanks in advance.