Mailgun emails not going through

Replies: 0

All settings including API key look good, all necessary DNS records added, domain is verified, ports are open, but I cannot send email from the domain.

Versions:
WordPress: 5.1.1
WordPress MS: No
PHP: 7.0.33
WP Mail SMTP: 1.4.2

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

Debug:
Mailer: Mailgun
Forbidden

Please tell me what the “Forbidden” refers to and how to change it?