Email from the internet is delayed between 0-10 minutes. I can see the message come in on the Edge server in ProtocolLogs-->SmtpReceive. I can see it depart the Edge server (for the internal server) in ProtocolLogs-->SmtpSend... but sometimes there is a delay between the Recieve and Send logs. And I see no reason why.
Both servers are Exchange 2013 CU11 on Win2k12R2. CPU is sitting at like 1% and memory at 1.9GB/6.0GB used.
Sometimes the mail is instant, others it is delayed a few minutes. Nothing bad. I never hear of anyone getting an automated "Your mail is delayed" message. But in this world of everything instant, they're asking for instant email.
Org is very small, like 10 people. Internal server is a single multi-role server. No DAG.
I see no errors in the Event Viewer and I never catch anything in a queue (Get-Queue).