Hi All
So it would seem I’ve not been the only person to encounter issues with sending mail via my ‘own’ SMTP server, which in comforting, in an odd kind of way.
My suggestion would be to approach the provider of your SMTP server… if you can be bear to read on, here’s what I found:
Contrary to my original thought, my issue was not resolved by removing and re-authenticating the SMTP server. This worked for one email, but after that, the original issue reoccurred.
It seemed odd to me, as the same SMTP server accepted and delivered my invoice emails (which were a forwarded CC copy of the email I’d received from QuickFile’s AWS mail server). For whatever reason, the SMTP host wasn’t delivering emails sent from QuickFile. There were no error messages and as far as the QuickFile system was concerned, the messages were being delivered. My tests didn’t concur, so I decided to approach the host for the SMTP server - Easyspace (IOMartMail), in my case.
Having logged a support ticket with Easyspace, they eventually agreed to track a test email sent from QuickFile through their system. They found that the email was being blocked as Spam. I couldn’t understand why, as when I sent that very same email via my Outlook client, it did get through, despite using the exact same authentication details in Outlook, as used in QuickFile and having the same content.
Easyspace were able to release my test email from their filter and the message was then delivered. Asking Easyspace why the email was blocked, they indicated that the message scored highly on the spam filter scale, as it had “Invoice” in the subject and a URL link in the body of the email. I asked if the message was being blocked due to it being sent from QuickFile - I was looking for a reason why Outlook worked, by QF didn’t - I was advised it was just the subject and URL, which I still seems strange to me. They did indicate that there had recently been a large number of spam messages picked up that had similar traits to my messages, which was likely why it had scored highly on their filter system.
Easyspace suggested that I stop using “Invoice” in the subject and remove the URL. This rather annoyed me, tbh. As their paying customer, I shouldn’t be dictated to as to what I can put in my legitimate business related emails. However, as a test, I did try removing “invoice” from the subject, but it still got blocked.
I approached Easyspace again and explained that since the email was to send an invoice and the URL was a link to that invoice, I couldn’t avoid including those elements. They then indicated that the issue could be resolved by removing my domain name from routing through the Spam filter. However, they warned me that if my domain was reported for Spam, it would automatically get blocked, without investigation and it would be up to me to get them to remove it. Frankly, I couldn’t see any difference to the situation I had already, so I agreed. I send marketing email via MailChimp, so I didn’t see that it would be an issue.
Easyspace made the change and a hour or so later, confirmed the completion of the process. When I sent a new test invoice, I was pleased to find that it did get through. I have since sent invoice emails to various customers and haven’t seen an issue.
I’d like to say I completely understand why this issue occurred… particularly why the same email content was only blocked when sent from QuickFile. Sadly, I can’t, but I am pleased it’s working again, as it was a proper pain in the backside having to send them out manually!
Good luck to all having the same or similar issues - I hope you find the answer for your setup. 
