This is an older thread but I'm following up with an issue and a solution, in case this is helpful for others.
Before I dig into the issue, I want to express that - at almost a year later, this has been the most helpful thread I've encountered on this forum. Me and my clients have received such a small amount of spam after making these adjustments. Thank you again for sharing this information, RMedure!
OK, now for the issue - and solution.
I have set myself and clients up to collect their email in gmail but had been running into troubles recently. I could continue to send and receive my own mail in gmail, but could not set a new client up in gmail. Here is the error I got from gmail during the account setup - SMTP settings:
> Couldn't reach server. Please double-check the server and port number.
Note: I collect my mail in gmail using my own VPS mailserver settings, not using gmail's mailservers.
I had no issue setting up the receiving server (POP) and collecting mail and no problem with both the sending and receiving servers in a local mail program, such as Mac Mail. Only gmail's SMTP server that was having the issue.
KnownHost tech support examined the /var/log/exim_mainlog file and found that the following line to be helpful:
> 2016-05-23 01:34:19 SMTP protocol synchronization error (input sent without waiting for greeting): rejected connection from H=[18.104.22.168]:32842 input="\026\003\001"
Tech support has helped me pin my issue down to the delay code I added from #4.
I adjusted the wait from 15s down to 2s but if I see an issue rise, I'll increase the wait and whitelist/allow gmail servers, instead. (Thank you Marvin C!)
I've pasted his response below, as it was helpful to me and may be helpful for others looking to collect mail from a service such as gmail.
Have a nice week, everyone.
We've narrowed it down a bit further; cPanel's Exim configuration will introduce a 15 seconds delay by default, and we disabled this but the delay still happened. It looks like there is some customization on the mail server that is attributing to this - the customization is:
# Do not enforce sync (and likewise delay) for these hosts
accept hosts = 127.0.0.1
control = no_enforce_sync
accept delay = 15s
which causes that 15 second delay. This is in WHM via Home »Service Configuration »Exim Configuration Manager in the advance editor.
As mentioned earlier Gmail is not waiting the allotted 15 seconds for STARTTLS, instead it tries to send data immediately so this gets a disconnect as shown earlier in the logs provided. In gmail settings SSL = TLS and that really means STARTTLS. Generally Gmail should be able to handle the 15 seconds delay but it is not.
This was edited to to 2 seconds for testing purposes and that allowed the gmail connection to work.