Soft opt-in
A narrow legal exception: you may email existing customers about similar products if you offered opt-out at signup and in every message.
Say it out loud
“There is a narrow exception for our own customers, and it has four conditions we have to meet all of.”
Soft opt-in is not “anyone who bought once, forever”. In the EU and UK it is a limited path that sits next to full consent, and it only holds if every one of its conditions is true. Purchased lists never inherit it, and neither do people who enquired but did not buy.
Or negotiations for one. An enquiry that went nowhere is thinner ground, and in the UK the regulator has said so.
Your own. Not a partner's, not a new business line the customer would not recognise.
At collection. Not later, not in the first email.
There is no partial credit, and condition 3 is the one almost everybody fails, because it happened years ago on a checkout page nobody has looked at since.
Yours
Nobody does this for you.
What goes wrong
Assuming it applies across the whole EU identically. It is a Directive, so each country implemented its own version: Germany's four-part exception in UWG §7 is not word-for-word the UK's PECR regulation 22, and B2B treatment differs sharply between them.
What people get told
The claim. “They bought from us, so we can email them.”
Actually. Only if all four conditions hold, and only about similar things you sell. A customer who bought a kettle in 2019 from a checkout with no marketing opt-out is not a soft opt-in, they are an unlawful send.
The dated rules behind this
A definition is not a citation. These are the pages with the primary source, the date it was published, and what to do about it.
See also
Where this sits
Stop 1, you get the address. Someone hands you an email address, and the terms of that handover decide everything after it.