A surprising number of people run into this problem:
"A colleague sends me draft emails as attachments to a message. When I open the attachments to revise the emails before sending, the send button is grayed out. How do I send it?"
To send the message, you need to move the attachment to your Drafts folder first. You can do this by dragging the attachment from the message to the Drafts folder. Once in Drafts, open the draft message and the Send button will be active.
On a similar note, if you want to resend a draft message multiple times, don't double click to open it. Instead, select it then hit the Forward button.