Apple Mail - my sent email marked as spam in client's inbox

Email sent from the same address, but sent from my iPhone, are being received as normal. It seems that only email sent from my computer (Mac Pro 2013, trashcan) are being marked as spam in my client's inbox. My email address is my own domain (my first name at my https domain dot ie) and I have been emailing some of these clients successfully for a decade. Can anyone help? My domain is not blacklisted. thanks

It seems that the problem was with my SMTP settings. I had to authorise my IP address in my website control panel, and in apple mail I had to turn OFF 'automatically manage connection settings', so that I could select the port, SSL, and authentication recommended from my web host.