There are different versions and interpretations of consent and that will become increasingly the case as GDPR is implemented.
A good start is this https://ico.org.uk/media/action-weve-taken/mpns/1625739/mpn-lad-media-ltd-20170118.pdf – paragraph 23.
it is linked to marketing and spam texts but the critical thing is what it is saying about how someone has agreed you can contact them.
You have to be able to demonstrate that someone intended for you to contact them or that what you have to say is really of interest to them.
Blanket statements are just not enough.