There is no way you can know if you email gets blocked by a filter, the filters do not let the senders email clients know, the filters are not going to ping that server anything, it will tell them absolutely nothing. That's part of the protection, it doesn't want to connect with them. As far as the senders email client goes, it thinks it made it. Your email not going through because the email is not available is completely different, you get a return email, and you can't know if my or anyone else email is full, for one thing it can't get full mine has unlimited storage.
You don't get to see the emails you sent to the recipient in its final stage, the recipient does indeed get ads. You are not seeing the final product. For you to claim IncrediMail doesn't send ads along with there emails is absurd, its a well know fact that it does, if your IncrediMail account is free, you are sending banner ads and other spam-like ads to everyone, that's assuming of course they ever received it, and there anti-spam blocker didn't remove it as spam.
Yes my forum sends out a Reminder email if you have that "option" enabled in your profile, and 2kb it sends is extremely small because its all text with 2 text links, one link to view the post and the other so you can click to not to receive the notifications, and no ads. Thanks for pointing that out how much more professional and smaller my forums emails are than IncrediMail.
However that didn't help your case any, it worked against you by comparing the huge difference of the average size emails sent between IncrediMail and this forum, IncrediMail on average is 80k and this forum is 2k which is massive in comparison, but its safe to say IncrediMail users can be much larger, as the forums are always consistently the same small size.
You're forgetting about the users that don't have a fast connection, they could literally have to wait several minutes to open your emails. Just because it loads fast to you doesn't mean it is for them, and lets not forget you are not seeing the final product. If you're gonna say you sent yourself an email and there wasn't an advertisement, then I would suggest that IncrediMail doesn't spam its own server by sending you one too. Other than that if you viewed one on any other email service it could easily have been blocked with that emails default spam filters. The reason a filter even has to be in place to stop IncrediMail's nonsense is an invasion.
The fact remains that IncrediMail does indeed send banner ads and other type ads with each and every of your emails you send if your account is free. You can't escape that fact.