Emails bounce for several different reasons, the first time you send to them, it could be a simple typo which means that they bounce. However, that is not the one and only time that they might bounce, even contacts who you send to on a daily basis through outlook/your email client, could still bounce.

Why is this?

Well there can sometimes be a number of reasons for this: -

1. The email address/mailbox is having issues and this is causing an error at the recipient's end of things, causing it to look as if the email address has been deleted

2. The Email address no longer exists, this could indicate that the person no longer works at that company

3. Your email has been marked as spam by the recipient's servers, even if you have sent out several emails to the contact previously, there is still a chance that your email might hit a few too many spam words which could then lead to it being marked as spam and effectively returned as junk 

There are other reasons for this but these are the top ones, so double check the email addresses and try and avoid spam words! Then you can have email success! :)