Hello @GavinS2 ,
Here's more information on the suspended bounce. If a contact you know is valid continues to bounce as non-existent from Gmail's servers, then I'd suggest going through some basic deliverability steps to add additional authentication for your emails to this particular contact.
Deliverability can have a lot of variables, from elements on our side, on your side, on the recipient's side, and in their email system company's side. It's also a generally good idea to set up additional authentication for your emails - whether that's through us if you have a free domain (e.g. Gmail, Yahoo, AOL, etc.) or via self-authentication if you have your own domain (such as through a business or other organization).
If you'd like to learn more about safelisting, and what it entails:
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Deliverability agents.