Since posting this I have done some further investigation of this problem - there is definitely a repeatable problem (at least in my hosted environment). This relates to the latest beta release 1.9.6 Beta1 running an a separate .Net framework 4 application server and separate web SQL 2008 server. As a newbie, not sure whether to post in the separate issues database - please advise?
Bear with me and I'll step through the test...
I created 4 test users (Test 1,2,3 and 4) posting to 4 different email accounts. I tested that the basic email functionality worked from YAF by using the admin function to post to all users - which worked fine. All 4 test users received an email correctly.
I created a new Forum and made Test 1,2 and 3 watch that forum for new topics. I then created new topics in the forum using a variety of those test users and monitored which of the other users received notifications. The results were as follows (bear with me on this for a little while :-))
Test1 posts a new topic. Test 2 and Test 3 receive notifications (as expected)
Test 1 posts a second new topic. No notification received by Test 2 and Test 3
Test 2 posts a new topic. Test 1 receives a notification, Test 3 does not.
Test 1 posts a new topic. Test 2 receives a notification, Test 3 does not.
At this point I made Test 4 also monitor the forum and continued the test
Test 2 posts a new topic. Test 1 and Test 4 receive a notification, Test 3 does not.
Test 1 posts a new topic. Test 2 receives a notification. Test 3 and Test 4 do not.
Test 3 posts a new topic. Test 1 receives a notification. Test 2 and Test 4 do not.
Finally...
Test 4 posts a new topic. Test 3 receives a notification, Test 1 and Test 2 do not.
Apologies for drawing this out, but this was important as the sequence is now fully predictable (and I tested this further predicting what would happen, which it did). Basically the first time that a user registers to watch a forum then that user will always receive a notification the next time that a new topic is posted. However they will NOT receive any further notifications until they post a new Topic themselves. The last user to post a new Topic is always the only user (apart from users who newly register to watch the forum) who receive a notification of the next new topic posted.
Something similar (I believe the same) is also then happening for notification of new posts within the topic, although i have not repeated the same structured tests on this as yet. Notifications of posting are, however, following a similar pattern and it is clear that not all users are getting notification of posts if they have signed up for this.
Hope this makes sense - I would appreciate if someone could log this formally for investigation and fix within the next release if you agree?
Huw