Symptoms

Webmail webmail.example.com shows the blank page or the default plesk page.

DNS records for webmail.example.com is managed by external DNS server:

# dig +short webmail.example.com @8.8.8.8
203.0.113.2

There is another IP 203.0.113.3 is set in web server configuration file for webmail.example.com:

RHEL-like systems

grep VirtualHost /etc/httpd/conf/plesk.conf.d/webmails/example.com_webmail.conf | grep -v "/VirtualHost"
<VirtualHost 203.0.113.3:7080 127.0.0.1:7080>
<VirtualHost 203.0.113.3:7081 127.0.0.1:7081>

Debian-like systems

grep VirtualHost /etc/apache2/plesk.conf.d/webmails/example.com_webmail.conf | grep -v "/VirtualHost"
<VirtualHost 203.0.113.3:7080 127.0.0.1:7080>
<VirtualHost 203.0.113.3:7081 127.0.0.1:7081>

Both IP addresses belongs to the server
Cause

IP address that is set for domain does not match the one specified on registrar’s side.
Resolution

Log into Plesk
Browse to Home > Domains > example.com > Web Hosting Access and change IP address to the correct one 203.0.113.2

If the domain has no hosting and used only to serve mail, Web Hosting Access link is not present in Plesk. However, to update IP for webmail’s configuration file is possible via command line:

Connect to the server using SSH
Update the IP address to correct one:

# plesk bin subscription -u example.com -ip 203.0.113.2

Source: https://support.plesk.com/hc/en-us/articles/115005104213-Webmail-shows-the-blank-page