Plesk server hosted in OVH SAS unable to send mail to Outlook or Hotmail recipients: Please contact your Internet service provider since part of their network is on our block list

Symptoms

• Plesk server is hosted with OVH SAS.

• When an email is sent to Outlook or Hotmail recipient addresses, the following error is found in /var/log/maillog:

  550 5.7.1 Unfortunately, messages from [203.0.113.2] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140).

• Recipient addresses from other mail services providers are not affected.

Cause

Recently, UCE Protect has placed over a thousand new ASNs on their blacklist. One of OVH SAS' ASN  has been listed on the UCEPROTECTL3 blacklist.

Resolution

Contact OVH to get a new IP that is not blacklisted or set up an External SMTP server in Plesk as per the article How to configure an external SMTP server in Plesk:

• For AWS SES it's required to set up your SES account, get out of the sandbox, get your SMTP credentials and apply the article above.
• For SendGrid it is required to adjust Postfix configuration as per How to configure Postfix on Plesk server to send emails using SendGrid