Howdy @Mariana !
That sounds like a pretty cool magic trick there. As far as I’m aware, inSided were planning on making this possible directly within the platform and did reach beta. However, it seems that the winds have changed and the Email Campaigns feature got put on hold for the time being. There were details about it in a couple of threads that might interest you.
It might come back someday, but it’s probably not likely to be released in the near future.
I’ve had a think about this for a good hour or so, and I’m not sure how easy it would be to set this up. But there is some advice about similar options.
It’s a bit of a tricky one because as a user, I might be cool with getting lots of notifications from a community that I’m highly active in, but not so keen for one where I only wanted to stop by and ask a single question.
Probably the best thing I can think of as a sort-of half and half option, would be to make clever use of the ranks feature. If you know roughly how long your users take to rank up, you could potentially set up a special rank that takes an average of 3-4 days to reach and then have that fire off a notification to the user when they unlock it. This way, if you’ve got users who are like the place and decide to stay, they should in theory get a (sort of) welcome/thanks for staying with us email.
When you set this up, I would definitely recommend turning off the notifications initially, while you’re still busy applying final touches and giving out the rank to your existing members. That way, you can help to prevent a sudden email tsunami bombarding the entire community at once. Make sure to double check that the queue has cleared out, and that all existing members have the rank before turning the notifications on!
On the other hand, users who only ever post one thing and then vanish forever won’t get flooded with emails that they’re not interested in as easily - which should also help to reduce the chances of being reported for spam by mistake. It’s kinda weird… But for mysterious reason a lot of webmail users seem to get the Delete and Report Spam/Junk buttons mixed up… I’ve never really understood why either… People just have this habit of treating Report Spam/Junk as if it’s Delete because it gets that email out of their inbox and out of sight.
I remember a time when a forum had sent over 1,000 emails to a user over many years, because the user was subscribed to hundreds of threads and never thought to turn them off. Instead of deleting said emails the normal way - which has no effect on your sender/spam reputations - they went ahead and reported every single email as spam… All 1,000 of them. As a result, the entire website got completely banned by Hotmail, Symantec, Gmail, Yahoo and a bunch of other anti-spam systems because the sender reputation was completely shredded. It took over six months to recover from that. The website later decided to disable the forum subscriptions and notifications features to protect the main site.
If you’re able to filter out that type of user, it will definitely help to reduce the risk there! Those who are more likely to remain engaged - especially those who love to help others - are more likely to respond positively. And if the only way to receive such an email is by actually engaging beyond the first two or three comments, that should take care of not upsetting users behind one-shot accounts.
Meanwhile, if you happen to reach out to the right kind of user, there’s a very good chance you’ll get their attention and they may well come back. You never know… That one user you managed to retain be your next Super User in a few months time! So you’ll probably want to bear that in mind. If I’ve reached this stage, it’s very likely that I’m enjoying the show and would love an encore.
As a very engaged user on my home base, I always love getting PM’s from the moderators because I get along really, really well with them and for me, it’s always exciting to find out what Tim has in store every day. Just like with that first welcome email, you only get one chance to impress. Use it well. 
And the ultimate advice I’d give as a user… Always respect my inbox and make sure what you send out isn’t just yet another automated mailshot! 
Sorry it’s a long reply! But I hope this gives some useful thoughts. While this might not quite be exactly the solution you’re after, I feel that it’s a pretty good balance that’s hopefully pretty close.