Do they want to be contacted?
You must check if the customers want to be contacted by e-mail or SMS, and give them the chance to object.

When you collect customer details, you must get their permission if you want to send them other offers or promotions.

You must always ask for permission if you want to share their information with another organization.

Letting customers opt out
Every marketing e-mail you send must give the person the ability to opt out of (unsubscribe from) further e-mails.

You must make it easy to opt-out – eg by sending a “stop” text to a short number, or using a “unsubscribe” link.

Emails or text messages must clearly indicate:
– Who you are
– That you are selling something
– What the promotions are, and any conditions

Source:
https://www.gov.uk/marketing-advertising-law/direct-marketing

Related links:
How to make a eDM
How to send a eDM
SMS out