Everything was working fine until today morning when mails to Office 365 domain did not got delivered. Upon investigation we found that every thing is working fine except for certain domain whose email are hosted of Office 365(**.mail.protection.outlook.com) the emailgateway stuck on line mail delivery queued. It does not even establishes SMTP connection to recipient domain. Tried to forcefully deliver all mail using deliverall and delivernow command but nothing works. Checked for blacklisting of our email gateway IP in office 365 but it isn't blacklisted. Contacted the recipient side and they told us that SMTP connection has not been established to their mail server(office 365). Checked for network errors but there are not any. Viewed Delivery Status in GUI and the domains where emails are not sent has host status set as Down. Tried to telnet form iron port to recipient mx and its is successful.
Update: Upon further investigation found the following error in mail log:
Info: Connection Error: DCID 101607 domain: xyz.com IP: x.x.x.x port: 25 details: [Errno 0] Error interface: y.y.y.y reason: network error
But checked connection to recipient mail server from email gateway and everything is fine telnet,ping,trace etc.
Update: Upon checking the domain debug logs to that particular domain
Rcvd: '220 C01FT018.mail.protection.outlook.com Microsoft ESMTP MAIL Service ready at xxxx
Sent: 'EHLO yy.yy.com'
Rcvd: '250-PU1APC01FT018.mail.protection.outlook.com Hello [x.x.x.x]'
Rcvd: '250-SIZE 157286400'
Rcvd: '250-PIPELINING'
Rcvd: '250-DSN'
Rcvd: '250-ENHANCEDSTATUSCODES'
Rcvd: '250-STARTTLS'
Rcvd: '250-8BITMIME'
Rcvd: '250-BINARYMIME'
Rcvd: '250-CHUNKING'
Rcvd: '250 SMTPUTF8'
Sent: 'STARTTLS'
Rcvd: '220 2.0.0 SMTP server ready'
