I have a server running plesk 10.2.0 with Qmail, inside we host a website and we also run our own mailing list.
For the mailing list we use the software ListMessenger 2.1.0.
We have no issues what so ever with the server, everything runs good even normal user mail.
The issue comes when we send emails to our mailing list which is a DB of around 20,000 people, ListMessenger generates batches and then send email using PHPMail and them emails get delivered to every inbox.
The problem is that it generates 20,000 emails and the email queue gets so big that our server lasts about 2 days to finish delivering emails.
So what's your recommendation to avoid the server getting such a load? should I get a bigger server? I currently have a VPS with 2GB RAM a 1000/100mb share port on the network and not sure about the CPU.
I have monitor theses stats when I sent emails and nothing gets over loaded so the only thing I can tell has too much load is the email queue.
You could try increasing the number of outgoing queues that qmail is using. By default it is normally 20 remote connections however I run mine at 60 to accommodate one of my clients running a mail list server.