Are you blacklisted?
If your website is hosted on a shared server… meaning your domains are hosted on an IP address with lots of other domains, your domain could be on email blacklists without it being your fault. If someone else on the same IP address is spamming, all the domains on that IP address could be blacklisted. If you get on a blacklist it’s usually temporary… but it can make getting your emails delivered very problematical for a while. If you know the IP address of the server your domains are on you can plug it in at this website and see how much email volume that IP address is responsible for…
If the email volume is high, and you’re not sending most of it, I’d consider moving from a shared server to a dedicated server. In fact, I’ve been on a dedicated server for many years and there’s no way I’d consider sharing my IP address with anyone else… it’s like starting up a business without knowing the other businesses in your neighborhood… a bad idea online or offline!
