> Could I solve this by setting smtp_helo_name in main.cf to > 77-172-184-9.fixed.kpn.net ? > Or is this a bad idea?
Yes you can set the helo name to match but this wont have much effect. Some servers are looking at domain -> IP -> domain which in your case is: mail.linetec.nl -> 77.172.184.9 -> 77-172-184-9.fixed.kpn.net Since mail.linetec.nl does not equal 77-172-184-9.fixed.kpn.net they might decide to reject or mark the mail as spam. The helo name has nothing to do with that part. Some mail servers will then do an additional check on the helo name to see if it also matches or at least is a valid FQDN. I have no idea on the percentage of servers that evaluate client FCrDNS. Having your IP listed as authorized in your SPF record for your domain could help in preventing mail being marked as spam.
