After jumping through hurdles to send email to gmail (basically restricting postfix to IPv4), I have recently been having issues sending to gmail with the above error. I can send to other domains with no difficulty, this appears to only be an issue with gmail. I have checked the DNS settings for the server and postfix and all are correct. I have also done a ton of searches with no solutions. Any help would be greatly appreciated! Edit: FIXED. The "smtp-ipv4" issue was found in the /etc/postfix/transport file. Check that thing if you're having similar issues!