We used to run our own incoming mail server on our internet.nl IP addresses. Since a while we use the MX's of Prolocation.

This means that no longer a mail server on port 25 can be detected on our IP's. However, some mail receivers and some RBL's do check for this.

For example, the IPv6 addresses of internet.nl and batch.internet.nl are currently included in dnsbl.spfbl.net with the status 127.0.0.4 which means "could not identify an email service running at this address, it’s a NAT router, or because it’s residential connection."
For more details: https://spfbl.net/en/dnsbl/
See also helpdesk ticket 711601

In the light of the above, it would be good if we ran a mail server on our own IP addresses.