Do you receive an email telling you that delivery failed? If so, what does it say exactly?
The other thing that causes this is SPAM filters on the recipients' accounts not liking something about your email address, your IP address, Yahoo! addresses in general, or some email server in the chain. If it's one of these then there isn't much you can do apart form ask the recipients to check their SPAM folders for your email or, if they are using email addresses belonging to a corporation, speak to their email administrator.