[Dovecot] version difference understanding

[upen]

Hello,

I am in process of setting up an IMAP server (with postfix SMTP), I
installed RHEL 5.5 on my server and found that version of dovecot is
1.x.x while when I checked online RHEL 6 is version is 2.x.x.

Can someone please tell me if these two versions are still in
development or 1.x.x is very old and must be replaced by 2.x.x
version?

Also are there any special features in 2.x.x or 1.x.x that makes it
better than other?

I see similar things for postfix package but I understand this is not
the place to ask postfix Qns,

Appreciate your help!
~A