When "replying" to certain emails, my reply is not received by the original sender. This seems to be a problem with certain domains -- I have identified 3 businesses. Replies to individuals (gmail, ISP's webmail, etc) usually get thru; some seem to be lost, however. There appears to be no problem with emails originating from me; only replies. As you might expect, this is difficult and time-consuming to confirm and troubleshoot, as it involves coordination with others.
When I reply to messages without the original message and header appended, the reply goes thru. This leads me to believe that the ISP or receiving domain does not like something in the message? I have contacted my ISP (Shaw in British Columbia), and they say there is nothing different that they are doing. My host domain provider also claims they aren't the issue.
Around the period when this began to occur (however cannot be certain of causality), I changed anti-virus and anti-spam vendors. Previously I was using ISP-provided F-Secure suite, but changed to MS Security Essentials and Windows Firewall to improve performance and random hanging (which was a substantial improvement). I also install Cloudmark freeware anti-spam (PC World recommended). I have uninstalled both, and disabled MS Firewall, and the issue remains. I also made sure that there is no additional user-controlled antispam settings enabled at my ISP or hosted domain.
I have now run out of ideas. A search of this site and Google shows others have experienced this, but no solution is evident. Any help you can provide would be greatly appreciated, and keep me sane!