[Dovecot] Webmail project : to cache or not to cache emails

[Vincent Richomme]

Hi,

I plan to develop a new web service where one of its module is a 
webmail and I am thinking about
some implementation details. From a interface point of view I wanted to 
adopt the same logic a outlook(desktop) or yahoo webmail
and not split emails into pages. I mean I just want a single datatable 
view where user can scroll to
see his old messages and data are updated only when user release the 
vertical scrollbar.
When I look at current webmail (roundcube for instance) I can see that 
a cache is used to stored
emails headers and I was wondering if it was really necessary ?
Once a user has passed the login process I would like to retrieve only 
the emails that will be displayed
(actually a bit more, I am thinking of 150) but I am wondering if it's 
a good idea to not use a cache for headers.
For instance if I have thousands of users on the same machine, will it 
support it ? Will it be fast enough ?
Of course I will try by myself the different options but would be 
curious to have some opinions.

Thanks