Helping resolving 'There was an error sending the email'

I’m using Mailgun for both newsletter and transactional mail for my self-hosted Ghost blog. It seems to be working fine when I publish a post and send it to my subscribers or trigger a preview e-mail for an unpublished post, but I receive the following error in my Ghost logs when people try to sign up on my site:

TooManyRequestsError: Too many different sign-in attempts try again in 2 hours

An e-mail is never sent to the e-mail after clicking “Subscribe” and this error appears in the browser console:

Failed to load resource: the server responded with a status of 429 ()

Any thoughts as to why mail might be working for newsletters but the same configuration doesn’t work for membership/transactions?

Edit: I’m running Ghost via Docker and am on v5.42.0. I see there are two updates (5.42.1, 5.42.2), but they don’t seem to have been pushed to the Docker registry yet, so I can’t pull them.