Hi, I have a qmail server (version 1.03) running on CentOS 5.3.
All the emails from remote addresses are delivered fine, but there is one remote email address in particular whose emails take up to 12 hours to get to the recipient. I can see in the SMTP log that it is comming in, but it doesn't go to the queue, and after several hours, the message gets delivered.
I did what you told me and I can see that the email is coming in in little chunks, sometimes taking several minutes for a little chunk to arrive of the same message.
If I ask the person to send a mail from the same email account to my personal Gmail account, I receive it immediately, it only takes too long when the user sends it to my server with qmail.
Worded differently it could be that your server and the server for the "one email address in particular" have a server network tuning or network device (router, firewall, load balancer etc) combination that are not playing nice together. It doesn't happen often but it happens.
Compare your results with the other end.
Is your server or their server generally waiting for a tcp response before continuing?