Hi Ken,
I have a particular interest in this thread due to some difficulties I have had with email failures which I suspect are down to my nameserver host.
You said you are an IT professional so let's just ensure we are on the same page:
You have your own domain, hosted with Namecheap.com. Your DNS records are hosted on their nameservers.
When you send an email from your domain it goes onto your domain's web server hosted on Namecheap.com. It then goes into a queue for sending on their server and attempts to send to the recipient's address. This has failed for toastmastersclubs.org addresses.
You have also said that you do not receive emails from toastmastersclubs.org addresses, however those emails did not get bounced back to toastmastersclubs.org or there would be a record on the toastmastersclubs.org system.
Additionally, Brian has has successfully sent email to your address that you have not received.
If it was me, I would be looking at my mail server at this stage to see what is logged on there. Your mail server is hosted so you would need to speak to the hosts.
...but as IT professionals we like to delve a little deeper, don't we?
mxtoolbox.com
IP address for your mail server is 199.188.206.63.
Blacklist lookup. The
IP address of your mail server is blacklisted on UCEPROTECTL2 and UCEPROTECTL3. This means that email would appear be intercepted in flight! Why? Because any mail server that uses blacklists to protect itself (and I hope that is all mail servers) would not even handle email from your server and the details of it would not appear in logs. What this also means is you don't get to see what fails. You only get to see what works.
There are two ways to deal with this. One way is to add your email server or email address to whitelists on recipients mail servers - this isn't a solution since your problem is global. The best way is to get your email address removed from the blacklists. That is the first thing that needs to happen.
Brian mentioned that the subnet that your mail server is hosted on is found regularly in blacklists. The reputation (and I use the term technically) of your domain is fundamental to the success of the services you run from it and is being damaged by the users of the organisation who hosts your domain or by individuals who are exploiting your domain.
If you speak to Namecheap.com they might have a solution. If I was a service provider I would configure mail servers to use SPF, DKIM and DMARC and suggest to clients that they tweak the settings as they need to. See
www.higherlogic.com/blog/spf-dkim-dmarc-email-authentication/
for more information. You currently do not have anything configured to validate your email address or server.
As a last resort you might choose to move to a different hosting company to avoid these issues.
