I think it is perfectly fine to do, as long as:
- You have not said you wouldn't do this - at sign up stage, in your terms, etc.
- You allow a way within the email that is simple and clear
to unsubscribe (and it actually works)
- It's not a huge load of nothing - It needs to be interesting, with
some good reason you are emailing, rather than just "please come
- You might not want to email all the same day. To avoid the site
grinding to a halt when a lot come back at once. This depends on your
hosting resources, and how many users you mean by "a large number of
users", and possibly the niche (non of which you mentioned)
- Depends on what niche your site is - some niches this wont work for
at all, some it will
- I would say email those who have: Posted more than 6 times (any
less could just be passing comments, hit and run question, reply just
to get sig links visible, etc) and inactive for 2 months or more.
Anyone not been to a forum in 2 months is not really active, and
after 6 months they are likely not to remember and thus not care as
much (but with 2 months threshold you catch them anyway)
- You could try limiting the mailing list to say 5% of your total list,
and see what feedback comes (again, I do not know how many there will
be, 5% may be 10 users, which us useless, or be 5000 users)
Those who see it as spammy will just unsubscribe/ignore/email some profanities/etc.
Nothing harmed as long as you unsubscribe those who requested it and don't email them again.
Of the ones I receive, I ignore many but I do revisit some sites. It feels like a grey area to me
You are of course just seeing your own actions, and you may find a surprising number of people respond to such things, like coming back and being active.
It does really depend a lot on your niche, what message you actually email, and how many users this will go out to, etc.
If you are stuck with a good reason to email them, and all attempts sound like "please come back we want more members", then perhaps think about starting a newsletter.
It doesn't have to be juicy new features, just the latest most interesting threads from a range of your topics/forums.
Then, something might really grab their interest, and if so their coming back might not be just another hit and run.
Again, because it's important, don't forget the link "Click here to unsubscribe to these messages".