I have created a custom buttom to send an email from a case. The problem that I've found is that I can get cases sent to 2 different email addresses. So when the button is clicked I want it to populate the from address (p26=) with the To address the email that was received, not just a specific one that is hard coded. Thanks for any help in advance.
I figured this out awhile ago, but just wanted to update this with the solution.
Parameters in the URL:-
p24 - Additional To Address
template_id - Email Template Id, It can have merge fields.
p3_lkid = whatId ( hidden lookup field) it is must if the merge fields are there in template
p3 = Related to name (input text)
p26 - From Email Address
p5 - BCC email address
save = 0, means send the email automatically and return to retURL ,( leave it if you just need to see the screen with populated fields)
I found this information at: Forcetips Url Hacks
Another resource I found was at: Salesforce Ben Email URL Hacks
I hope this helps someone in the future.