First, just for fun, try restarting the entire server. See if that makes a difference.
If that doesn't work, run the following:
Login as root:
sudo -i
Edit mail configuration:
cd /var/www/new/ehcp/webmail/config
./conf.pl
Pick option 2 (Server Settings)
Pick option 1 and set the domain to your main domain
Pick option 3 and make sure it is set to SMTP. If that doesn't fix your issue, try using Sendmail here.
Save data by typing in S
Quit by typing in Q
service apache2 restart
service postfix restart
service ehcp restart
Also, RoundCube should be available. It's available at
http://localhost/ehcp/webmail2/ (from the server).
If that still doesn't work, check into your IMAP settings and make sure the IMAP service is running.