A customer contacted me with a major problem: when they sent an email to another party, this email was almost always mistaken for spam. And that also happened if the sender was already known to the recipient. This caused a lot of communication to go wrong.
After investigation, it turned out that the IP address of the customer, who has his own mail server, has a bad reputation on the internet. Presumably because the previous user of the IP address did not take security seriously.
The problem was solved by having the own mail server use an IP address with a good reputation.