One of our business contacts has trouble receiving email from us at his domain. It appears that no one in his domain (.com domain, not AD) receives email from us (at least not regularly - they may receive mail sometimes, but less than 50%). There are no error messages received in Outlook. Other users sent the same message receive it with his domain's email address in the to: list, so I know it is being sent, it just never gets to him (or his secretary, or his co-workers, etc.).
I've checked the email address in the address book and it is correct. I've looked at our MX & PTR records and they appear correct. I've checked our IP against blacklists (using mxtoolbox.com and a couple others) and we aren't on them. Our web hosting provider (all our email goes through them - and their IP is the one I checked for blacklisting) has assured me that if Outlook doesn't report an error, it has gone through, and they can't track it after it has left their servers.
This is a very important business associate, so I really need to get reliable email to/from this person's domain. If anyone can offer some suggestions on where to look next, or who to contact, I'd appreciate it.