We have solved the problem. We can send emails to gmail clients again. We are currently still testing, because our forwarders and autoresponders are not working as normal. But you won't notice that.

We have a temporary fix. We can now only receive from all clients. Sending is not yet possible.

We are changing the mail server. There is a chance that the mail server is not working completely. Due to DNS record changes.

Gmail.com sees our IP address as spam, so we cannot send emails to gmail.com, but we can always receive them. We are working on changes so that we can send to everyone.