If you make appointments with people you know or if you have collected contact details by specifying the purposes of your processing via opt-in forms (your contact explicitly ticks a box authorizing you to process his data), then you have the right.
If you have recovered databases of email addresses without the consent of the persons concerned (scrapping, spam or other techniques that are less than questionable) then you are illegally processing data vis-à-vis the GDPR and expose yourself to possible sanctions.
Consent to data treatment is the basis of the GDPR.