Mailgun failed (Forbidden)

I can add the observation that all of the sites encountering this are ones that I have migrated to new servers (though, in the same Digital Ocean environment). I expect either something is improperly configured on my server, or the Mailgun API has somehow cached the old IPs connected with my domain and are not properly communicating. Any assistance would be greatly appreciated! Thanks!

Versions:
WordPress: 5.9.2
WordPress MS: No
PHP: 7.4.3
WP Mail SMTP: 2.5.1

Params:
Mailer: mailgun
Constants: No
Api Key / Domain: Yes

Debug:
Mailer: Mailgun
Forbidden