Until the ePrivacy Regulation is enforceable – expected to be in 2020 (date to be confirmed) – The Privacy and Electronic Communications Regulations (PECR) will continue to be the prevailing law governing electronic communications and messaging , including email, and this includes the provision for soft opt-in for marketing to existing customers.
If a company has obtained the contact details of the recipient of that electronic mail in the course of the sale or negotiations for the sale of a product or service to that recipient and the direct marketing is in respect of that person’s similar products and services only and the recipient has been given a simple means of refusing, then there is an option to use a ‘soft opt in’ as your lawful basis for processing. However, you must have given the recipient a clear chance to opt out – both when you first collected their details, and in every communication you send.
The soft opt-in rule means you may be able to use electronic messaging to communicate with your existing customers, it does not apply to prospective customers or new contacts (which require consent).
Further details on PECR the ICO guide to PECR can be found here: https://ico.org.uk/for-organisations/guide-to-pecr/electronic-and-telephone-marketing/electronic-mail-marketing/