cURL error 51: SSL: no alternative certificate

It seems to be an error on Mailgun side, I would contact their support with this message:

cURL error 51: SSL: no alternative certificate subject name matches target host name ‘api.mailgun.net’

They might have changed the SSL, or it’s expired, or they made some changes in their infrastructure, I’m not sure.

But that’s definitely not a plugin fault.

For your reference, I received this reply from Mailgun:

Marco Salazar (Mailgun Support)
Jul 27, 1:32 AM CDT

Thank you for contacting Mailgun support

The Root Certificate Authority bundle (CA Bundle) will need to be updated on your server. WordPress uses a local cache instead of the system root certificates. As such, these would need to be updated periodically or rely on WordPress’ routine updates to include newer root certificates. To update the certificates, download the updated ca-bundle.crt from haxx.se and upload the file to the wp-includes/certificates/ca-bundle.crt directory.

Reference: https://curl.haxx.se/docs/caextract.html
https://www.tylercruz.com/having-wordpress-trouble-due-to-v4-4-heres-the-fix/

Please let us know if you have any questions.

Thanks,
Marco@MG

The Mailgun preset configuration choice in WP Mail SMTP no longer works. It results in the cURL error.

However the WP Mail SMTP generic “Other SMTP” configuration choice allows manually entering the necessary items for Mailgun emails to work without error. This is an acceptable workaround for now.

Mailer: Other SMTP
SMTP Host: smtp.mailgun.org
Encryption: TLS
SMTP Port: 587
Authentication: ON
SMTP Username: “username”
SMTP Password: “password”