Mac Mail emails being quarantined

I'm having major issues with my email at the moment. Some of my emails are being quarantined and not reaching recipients. Some are not. I've generally found my emails are being quarantined when i send them to bigger businesses as opposed to personal email addresses. They are showing as sent in mac mail but are not reaching the recipients.

I have tried deleting my account on mac mail and setting it up again. When setting up again, when i enter my email and password i get a message saying "unable to verify account name or password". I can then continue and set up the mail box as normal and it seems to work. I have contacted my hosting company, 123 reg, and they don't seem to think theres anything wrong their end.

Can anyone help me out?