The GDPR makes a distinction between two different type of emails:
Without commercial aspect: holiday greetings or best wishes emails, information concerning the GDPR, etc.
With commercial aspect or direct marketing: includes price information, discounts, promotional offers, etc.
For emails without a commercial aspect, you only need to offer an opt-out (i.e. the possibility to unsubscribe for these type of emails) in every email. Obtaining someone’s consent or permission will be important for emails with a commercial aspect. For example, you will need people to opt in for commercial emails, instead of sending it to all your contacts. Only with people’s opt-in will you be able to send direct marketing. The possibility to opt out is required here too.
There are two cases that don’t require a consent:
If you send direct marketing to existing customers and the content of the email promotes similar products or services offered by your company. Bear in mind that you will always need to offer the possibility to opt out.
If you send direct marketing to legal entities (firstname.lastname@example.org) and it relates to products or services for legal entities. Again, you will always need to provide an opt-out.