Bounces are determined by the receiving server, and it means the server rejected your email. It can reject for any number of reasons, and it doesn't necessarily mean the email is bad.
It can also be confusing as a 1:1 email from a sales person might go through, but a marketing email does not. This is because the receiving server knows if this is a direct email or an email sent to an email list (mass marketing email). Some things that could cause a server to bounce include:
Invalid address
Closed inbox
Limits on size of attachments
Limits on size of images
The wording in the email
Strict fire walls blocking unknown IP addresses
Strict rules regarding emails sent via SMTP relay
For the best deliverability
Avoid "spammy" words of phrases; our Email Tests in the email builder can help identify those
Limit the size of your email to 10MB at the max (some servers will be stricter than this)
Limit the use of images to 1-2 images; some very strict spam filters will trigger if there are more
Avoid sending mass marketing emails too frequently (more than once per week)
The best way to guarantee deliverability is to have your recipients whitelist our IP range. We realize you won't always be able to do this, but it will make sure your marketing emails get through. Here is our range:
IP Ranges:
87.253.233.100 to 87.253.233.250
87.253.234.100 to 87.253.234.250
87.253.238.217 to 87.253.238.2241
85.189.236.11 to 185.189.236.52
Prefixes:
87.253.232.0/21 and 185.189.236.0/22