This script builds dovecot with support for SSL. As such, you must have
openssl installed - openssl-solibs is not enough in itself to meet this
requirement.  Additionally, dovecot will be compiled with POP3 support.

You should create users and groups for dovecot and dovenull to run the 
login processes on your server. Something like this should work:
[root@host] # groupadd -g 202 dovecot
[root@host] # useradd -d /dev/null -s /bin/false -u 202 -g 202 dovecot
[root@host] # groupadd -g 248 dovenull
[root@host] # useradd -d /dev/null -s /bin/false -u 248 -g 248 dovenull

From the dovecot-new release posting comes this word of advice:
  Remember to read http://wiki2.dovecot.org/Upgrading/2.0 before
  upgrading from v1.x. Although there is automatic config conversion,
  it can't handle everything, so reserve some extra time for upgrading.

See the documentation in /usr/doc/dovecot-*/ to get started.