The easiest solution is to have the receiver’s “white list” you. The easiest way with most webmail providers is have them add you to their address list (the idea being that if you’re on someone’s address list, they want to see mail from you). The same kind of thing can apply to other mail services but each one of those can be a bit different.
It could also be occurring because the mail server managed to get caught in a couple companies blacklists. DH support needs to know so that they can start fighting.
You may also want to stop saying words like Viagra and Cialis in your mails.
There’s no certificate involved in this process. Short of the content of your mail, you have no control over how someone’s mail system handles it.
Others may have more to say, but I know you’ve waited patiently for some kind of answer.