Here another big blacklist check: https://mxtoolbox.com/blacklists.aspx

Can you help

One group of clients at a business i correspond with regularly has stopped receiving emails. they have (as an example no real name used ) johndoe@XXX.org email addresses. No -one here gets email from me. I can send to anyone else without problems but not this group. I discovered it after I had replied to an email about a meeting. I got an urgent email asking if I was attending. I had replied, and to several other emails regarding the meeting but they did not and still don't receive emails I send. 

 What could be causing the problem?

 I get no error messages anywhere, and emails are marked as 'sent' in the sent folder of PM and in gamil.. 

Sounds like your mail domain has been added to a mailing blacklist with this enterprise. We are experiencing this from time to time. You could check different worldwide blacklist services whether your MX IP or Mail domain is listed there or you could ask the IT of the affected client group to check whether they are filtering your mail. This could happen also without replying an error message to the sender, to avoid that spammers get any reply from a mail server.

Thank you that is helpful, how would i check this blacklist service?

Try Surbl   at http://www.surbl.org/surbl-analysis  You will need to lookup the Url to get the IP address first.

Martin 

Here another big blacklist check, which is asking different blacklist server: https://mxtoolbox.com/blacklists.aspx. Do only enter your domain name, i.e. "company.com"

Have you asked them to check their spam folders?