If a customer doesn't opt in for your mailing list, are you still ok to contact them via email in the future? Obviously you wouldn't add them to a regular mass mailing list without their permission, but can a sales person follow up in 6 months to see if they were happy with the service for example?

Best practice is clearly to get them to opt in, but are there actually legal issues with the above?

Thanks for any help and advice you might be able to offer.

With a disclaimer like Sonia that I'm not a lawyer and not therefore giving legal advice you should trust... ;)

But, yes, you can email them. You should check the wording but, for business customers in particular, you can contact them if you've sold them something or even if you in negotiations (or ever have been) to sell them something. So you don't need opt in if you've already been in sales discussions and they've given their email details. 

But of course it's not just about the legal technicalities. More about whether they actually *want* you to contact them and what the impact is in *their eyes* (as opposed to what the law might say). If they mark this contact as spam... how might this damage your overall sender reputation and possibly damage *all* your email deliverability etc.? This, I think, is the bigger consideration.

I think it's worth re-iterating Ashley's point.  Although it might be technically legal for you to contact a database of people who have evaluated your product but not purchased, and maybe even contact them up to a year later (you would need to double check that point), you need to consider whether it's really worth it.

Any email recipient can still report you for spam if your email is unwelcome, and this will not do much good for your reputation with either your prospects or the ESPs - potentially ending you up on a black list.

First and foremost you need to consider whether your email would be welcome.  If not, then is it really worth hacking people off?  

If you think that your email is genuinely going to add value to your recipients then it might be worth using the email subject line to make it clear that you are following up on their sales enquiry.  It's not foolproof but it might help?