Can't send mail on iPhone 12

I recently upgraded to an iPhone 12 mini, and restored everything from backup. In the Mail app, I now can't send mail (I'm receiving mail just fine). I'm using exactly the same account settings as before, which are also the same account settings as on my iPad. I was initially getting a notice that said the certificate from my provider was invalid; I've had this issue in the past, which I'd resolved by deleting my email account from my phone and adding it back. I've done that, but now when I try to send an email I get a popup that says "Cannot Send Mail. The server rejected one of the recipient's addresses".

Has anyone else experienced this? Any idea of what else I can do?

My email is hosted at Dreamhost. I'm running iOS 14.8