If you forget to add a subject to a mail you want to send, you cannot send your mail after filling out a subject.

Consider this scenario:

• Open the contact page of a contact
• Click on an e-mail address of a contact
• Compose an e-mail without a subject
• Click 'send e-mail'
• You get an error message that a subject is required, and the 'send mail' button is greyed out.

So far so good, this is expected behaviour.

But if you now enter a subject, the 'send mail' button stays greyed out. I guess this is a bug.