SES 554 Error on verified email

I have two websites (on lightsail) that use SES. The first is running perfectly, and messages are getting through correctly.

With the second website, I get 554 errors that the identity has not been verified.

I'm using the same endpoint, the same SMTP username and password, the same connection settings. It's authenticating successfully, so everything to that point is fine.

I've checked the identity list, and the email I'm sending from is coming up as verified (in the correct region).

The account is not in sandbox mode.

I'm slightly at a loss. I've gone through the docs thoroughly, and followed every step. Am I missing something obvious?