Long delays receiving mail from Google Mail hosted domains

Problem If you have been having difficulty with delays receiving mail from people that use either Gmail or have their domain Google Mail hosted, then you might like to have a look at your firewall and see if it is blocking the incoming mail. We have been having a great deal of problems with mail being delayed on receipt, and in some cases this can be minutes growing to hours. In one case exceeded the 2 day limit and bounced back to the sender. In all cases we had no record of the mail being anywhere near our servers until it was actually delivered to a mail client, usually seconds after being recorded hitting our servers. We used the incredibly useful Analyze Headers feature of MXToolbox on delayed mail and found that in every case it came through a Google mail server. What was most alarming was the indication that it was on a blacklist! Hop Delay From By With Time (UTC) Blacklist 1 * 10.25.225.211 HTTP 6/23/2017 8:24:31 AM 2 59 minutes mail-lf0-f54.google.com SMTP 6/23/2017 9:23:56 AM 3 0 seconds mail-lf0-f54.google.com 209.85.215.54 HE1EUR02FT018.mail.protection.outlook.com 10.152.10.248 Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P384) 6/23/2017 9:23:56 AM So we then ran the mail server through the RBL checker on MXToolbox and low and behold: Not all blacklists had been fired, and it seems that SORBS was used by our SonicWall firewall as a RBL checker, which was installed by default. Resolution We understand that there have been issues recorded with SORBS. Therefore we looked up what to change these to on our firewall, and picked the following as potentials with the help of...