That's the problem. At least 1GB memory is required for a full-featured iRedMail server.
If you have to use this 256MB RAM VPS, please try to disable spam/virus scanning (Amavisd+SpamAssassin+ClamAV) completely by following this tutorial:
http://iredmail.org/wiki/index.php?titl … amAssassin
Again, disable spam/virus scanning is not recommended for production use.