We run a small business with our internal mail server and a fortigate FW.
When clients send us emails using their free Gmail, some of these Gmails are received in by us, delayed from 10 minutes to 72 hours.
I have analyzed the headers for these delayed Gmail received in on Google admin tool box.
The GMail servers IP address the delayed emails come in from are in the range 18.104.22.168 to 22.214.171.124
GMails originating from these GMail IP addresses are literally stuck in the GMail server. Once they are sent from the GMail after spending the delay time on the GMail server, we receive them in about 1 to 2 seconds into our system.
We have gone outside our scope and have whitelisted all GMail addresses' as wild card, we do not check the sending ip address on any RBL list.
We have also whitelisted the ip addresses listed above.
We have tried using our staff Gmails addresses from their mobile phone ( 4G) , from remote work places etc. this is the same result continually.
Almost all of the IP addresses listed above are in one or more RBL lists ( however we do not check any RBL lists )
We are taking the onus to work this out, since these free GMail user clients of ours are small shop owners and individuals who are our business clients.
Seeking any ideas/suggestions, other possibilities to try, to overcome this annoyance.