I think there is value in just being able to do easy MUA configuration
for an email address. Something based on DNS is going to be easy to
deploy. SRV RR are more widely deployed and supported, that is why
draft-daboo-srv-email is using them.
But what is the motivation to implement and deploy the SRV record scheme?
An MUA can do better (better coverage, easier deployment) by probing
several common setups until it finds one that works. Outlook 2007 already
does this (sorry, can't find a more detailed description than
<http://support.microsoft.com/kb/287532>) and it looks like Thunderbird
has something similar in hand. All it requires from the operator is to use
common host names for the various servers. So I think this draft does too
little too late.