Sorry, it is not a spam filter. Although I do have a spam filter, it is not being dropped in that bucket either.
If it means anything, I have email notification on other websites, and I do not have any problem with them.
And like Dave, I feel I am being rude when I do not respond quickly. Like this post, I have to check all of my posts to see if anyone responded, so I can reply. (hence the 5 day lag in responding to your reply)