Hello,

Im running Exchange 2013 on Server 2012R2, and all has been fine for a while until some users complained that ome of there external recipients are not receiving their mails. if I look in the mail que I indeed see the messages stuck with the comment:

[{LRT=14-4-2015 14:36:14};{LED=441 4.4.1 Error encountered while communicating with primary target IP addre
ss: "Failed to connect. Winsock error code: 10013, Win32 error code: 10013." Attempted failover to alternat
e host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate
hosts.

Initially i thought it might had to do with the PTR but I checked and the reverse lookup is fine, also the mail server ip's are not blacklisted. I've tried every suggestion on all forums I could find from changing MTU sizes to disabling TLS but nothing seems to work the majority of the mail goes out just fine but to a couple of domains it doesn't any input would be greatly appreciated.

I have only the internal DNS server configured on the NIC, only one NIC is available and OWA, Outlook and Activesync clients work just fine.

http://public.wsu.edu/~brians/errors/their.html

If this is happening when you try to send to several domains, then I suggest you might be blacklisted or otherwise being considered a spam source.

Do you have an SPF record?  Is your IP address in a blacklisted range?

Hi thanks for your response,

Our IP is not blacklisted I've checked with mxtool and on the cisco websense portal.

I do not however have an SPF record setup, could this be something the receiving mailserver would check ?