If you need just webmail - you can use any PHP one like SquirrelMail if you have apache + PHP running on your server. But please note that such webmail software use IMAP to commect to mail server. So if you have no IMAP server now you'll have to install it. We can recommend you to use MailEnable for mail services (SMTP, POP, IMAP), then install apache + php and SquirrelMail for webmail.
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