Re: Suddenly unable to send email to 1 address @idnet.com
Update for others following this thread is that it looks like at least one of the Blacklistings for the relay 18.104.22.168 has been cleared because I was able to email my friend again on Wednesday 28th May.
However, when I checked on mxtoolbox blacklist search ( https://mxtoolbox.com) it is still on the Sorbs spam list though so may be being blocked still by other ISP's
I sent an email to my work address and it looks like the relay may have changed to know-smtprelay-omc-5.server.virginmedia.net (22.214.171.124) however, this is also on the same sorbs spam list as above. I assume they probably have lots of relay IP's/addresses.
Also thanks @ravenstar68 for the reminder to check on mxtoolbox.