- Plesk for Linux
1. IP address of a domain example.com is getting blacklisted constantly by spam filters.
2. Option Send from IP addresses is selected in Plesk > Tools & Settings > Mail Server Settings
example.com and hostname of the server
hostname.example.com resolve to different IP addresses:
# dig +short example.com
# dig +short hostname.example.com
Choose one of other options for Outgoing mail mode in Plesk > Tools & Settings > Mail Server Settings.
According to official Plesk documentation:
"Send from domain IP addresses - Selecting this option may result in mail sent from some or all domains being marked as spam if the Plesk server host name fails to resolve properly, or if the domain's IP is different from the one to which the Plesk server host name resolves."