Well, the good news is that you're not listed in any major blacklists (
clicky).
I have never heard of smtp.mailchannels.net before, I'm assuming its the mail service provided by your webhost; if so, you might try a separate SMTP service such as GMail (I use GSuite on both
A Gardeners Forum and
UBBDev to see if they detect any issues with your content and delivery to your users.
It's important to note that a 3rd party mail server can block you for any number of reasons, including sending notices too quickly; its their service to customize as they please, and that can sometimes leave their clients having to deal with it.
