Every now and then we get reports that our emails are delivered to a user’s
spam folder. We do have an SPF record stating what servers are allowed
to send email from -addresses but for many large email
providers that is not enough. Particularly Google and Microsoft are increasingly
picky. Being an adult oriented business doesn’t seem to help as many filters
see that alone as a negative criteria.
Last time we checked this was not possible with our MTA (email server).
But now there is a new filter plugin and we have been able to finally add
DKIM signatures to our emails.
This SHOULD improve the situation from now on. The signatures clearly
verify that our emails are from us and nobody else but us.

Sadly there is not terribly much more we can do to help here.