Tested:

1) Sending to a gmail account from SM server email account - works

2) Sending to an email on my SM server from a hotmail account - works

3) Sending to an email on my SM server from an outlook account - works

4) Sending to an email account on a fasthosts server from a gmail account - works

5) Sending to any email on my SM server from a gmail account - never arrives, nothing in the logs, no bounces, no errors in gmail!

6) Blacklist check - clear

So it's only the gmail emails that aren't arriving, and I have no way of telling why as there are no errors or anything in the logs.

Any idea how I can resolve?

I looked up your account to see what version of SmarterMail you are running and see you have  older versions and a couple newer versions running.  Which servers are seeing this behavior?  

Sorry yes, i'm currently on Build 8251

I tested some emails with build 8251 and no issues.  We have one other customer that just reported this to us as well and they are on an older v15.x release. In both cases (yours and theirs), SmarterMail had not been upgraded or touched.  In their case, we see no sessions from Gmail ever getting to SmarterMail.  This was leading us to believe there was a network issue, change, or dns problem.  I believe Andrea started a ticket with you to diagnose further but something at Gmail must have changed.  We are currently unable to reproduce but believe you are not receiving emails at times from Gmail. Hopefully we can help narrow down who or what is the culprit.   

Could you trying flushing Google's DNS for the domains that you think are being impacted.  We are seeing reports of others (not SM customers) having similar issues and this being the solution.

Flush Google DNS

No difference, unfortunately, i've just received a message from one of my clients clients saying they are getting a TLS negotiation failed message.

I think I've found the issue, I've recreated the .cer file that SM uses and it's now working.

Either that file was out of date or corrupt or something.

Anyway, I can now send to the server using my Gmail account.

It's still strange that there's very rarely a bounceback to say why it's not working (the one above was the only one I know of that was received)!

Could do with having something in the logs to say that it's failed to communicate with google or something?