Our company has sister companies that use our same account. Their domain is different than ours but both constant contact emails need to be trusted. The ones from their email address need to go to people's inbox and not their junk folders. It's not on the office 365 side because we've all added CC emails as trusted.
Hello @KayleighW26 ,
I'd recommend reaching out to our Deliverability team on this. While normally I'd recommend self-authentication, which their team can assist with, if you have two wholly different domains then the issue may be more complicated. Deliverability is going to have the best insight and tools to troubleshoot this, and assist with trying to get the other address's emails to go through.