Microsoft has introduced one more way for configuring autodiscover that does’nt require a complex configuration & certificate for autodiscover.
Outlook 2007 SP1 & aove query for autodiscover srv record on the DNS as well apart from other autodiscover queries.
So just create a SRV in the external DNS
point it to A record of your webmail (mail.domain.com)
After this when you will configure outlook it will work like a charm without needing any compex configuration on exchange side.
for more info refer to below KB article from Microsoft:
I have implemented this for one of our clients that is using Exchange 2010..& I am satisfied with the simplicity of its implementation.