Why are email recipients not receiving their outbound emails from SF?

Many of my users are expressing complaints

#Email #Bounced Email that they are sending timely quotes out to customers and they aren't receiving the emails. What could be causing this?

A few reasons I think, Security firewall from the recipients, or opted out from our Pardot email blasts? 

I put an Opt Out check box on the contact record (from our pardot blasts) in SF but I feel like that could be overlooked by the users. 

Also, Would there be a caution symbol by the email if the email bounced?