[AB] IP Blacklisted: Where is the blocking being done?

This may be easy for some people but it not for me. For years I have had SHAW as my ISP. Also for years I have had a web-host and a domain (www.domain.com) and have sent e-mail (me@domain.com) from it's server (mail.domain.com). I have also had other domains with the same host and sent e-mail from mail.otherdomains.com. This has worked flawlessly until suddenly e-mail I send from me@domain.com doesn't reach any recipient outside domain.com domain. No warnings, no bounce-backs, the message just disappears. E-mail sent to someone within the domain.com domain reaches the recipient. Actually some mail does get sent and received. Only mail with nothing in the body reaches the recipient. If the body contains text the recipient never receives it. These are the things I know. 1. My IP is blacklisted as per mxtoolbox.com (I am trying to get off but having trouble). 2. Someone else (someone_else@domain.com) using a different ISP than SHAW can send e-mail using mail.domain.com and it reaches the destination outside fred.com no problem. 3. IF I use a VPN, e-mail sent from me@domain.com through mail.domain.com does reach recipients with domain.com addresses but doesn't reach the recipient outside of domain.com UNLESS there is no text in the body. Even then someone@gmail.com doesn't receive e-mail which someone@shaw.ca does. 4. IF the body is empty the recipient usually receives the e-mail but never does if the body contains text. My web-host tells me it's SHAW's problem. SHAW tells me to request a de-listing from spamhaus. Since my IP is dynamically aligned from SHAW and I'm not running a mail server my only option on spamhaus seems to be turn on SMTP Authentication - which has been on forever. I cannot request a de-listing from them because I'm not running a dedicated mail server on a single IP address. So I'm not sure where to look next. Where is the blocking occurring and how can I fix this? How can I narrow this down to figure out how to address this issue? Thanks to anyone who might have any idea.