There are different reasons why an email can bounce. There are also different categories of bounces known as hard and soft bounces.
In order to send emails, you need to obtain contacts. One of the best ways to do this is by having a sign-up form or having a landing page, that allows people to put their information in.
A lot of bounces occur simply because your list of contacts is old. If your list is old, (more than 6 months since you sent it) then it's highly likely that many of the addresses have gone stale, been closed, or are otherwise inactive and will cause you to see a lot of No Mailbox bounces.
It's easy to include a request for updated information or to verify information right in the emails that you send, particularly if you're sending transactional mail. This way, you can potentially get ahead of the bounce by changing the contact's email address before it becomes invalid.
This might seem obvious, but it can actually sneak up on you if you're not paying attention. Spam filters are basically there to judge whether or not your email looks like spam.
You can use tools like mail-tester.com to help you stay on track or understand what changes to format, wording, links, or content will make sure your emails aren't treated as spam.
The more consistent your mailing schedule, the less likely your subscribers are to forget that they agreed to receive mail from your business or group.
This doesn't mean you need to send mail every day, but depending on the engagement patterns with your mail and the specific wishes of your unique contacts, it could mean that you have a regular campaign agenda so your mail is expected by both recipient servers and your subscribers.