A common reason for an email bouncing is a prospect entering it incorrectly on your website’s form or popup. To see if this is the case, try this:
Click on your welcome flow, then click on ‘view all analytics’
Next click on recipient activity.
Then, click on other → bounced.
This will give you a list of email addresses that bounced. A quick glace will tell you if there was a typo, such as emails ending in .co or .coms etc..
If you see email addresses that appear to be incomprehensible or are a company, these are likely bots fishing to get your discount code for discount aggregator sites.
The best way to prevent this is with a double opt-in. Otherwise, it’s really up to the quality of the traffic being generated to your site for your welcome flow.