The way I feel about email domains is that... as long as there's interent... there's a demand for email domains.
Whether people are buying the domains ie. EyeballMail.com, to use them for mailing purposes, or signing up for mail ie. hotmail.com - they're selecting something that fits their personality.
Something that jumps out and says... yep... it's me alright
So, as far as a valuation for EyeballMail.com... there IS great possiblility... I agree
$50 for the domain as is
$250-500 as a developed mail site w/o members
$xxxx and up for a developed site with memebers