Mac OS 10.5 Mail app hints - additional header area fields.

By default a new message has only two fields - 'To' and 'Subject' and it's perfectly enough for 90% cases when you have your reply-to address and signature pre-defined in your email account settings and basically you don't need to change anything in your message header fields.

Now when want to change signature for different messages, reply-to address or even smtp server to send a message from, you need a bit of customization and here is what you need to do:

1. Click on the small square field in front of a Subject line and click on 'Customize' and now by checking these fields you can specify which fields you want to have in hand for all your new messages. 
2. CC, BCC, Reply-to and Priority fields are on the list next to 'Customize' item on the same pop-up menu.

To specify different signatures you need go to 'Preferences --> Signatures', to specify different smtp servers to send your messages from go to 'Preferences --> Accounts --> Choose account you want to specify servers for and then click on drop down list at 'Outgoing Mail Server (SMTP): ---> Edit Server List.