Custom Send Email button on custom Objects

Hi,

If you want to add a custom button to send email on custom object then follow below mentioned steps:

1. Create Custom button on custom object;

2. in the content source, select URL;

3. in the value section, add following url

https://na15.salesforce.com/_ui/core/email/author/EmailAuthor?p2_lkid={!Estimate__c.Bill_To_NameId__c}&rtype=003&doc_id=00Pi0000000OluX&retURL=/{!Estimate__c.Id}?srPos=0&srKp=00T

where Estimate__c.BillToNameId__c is the contact Id to whom you want to send email

and

Estimate__c.Id is the return URL of your custom object

Reference: http://nanostuffs.com/Blog/?p=1341