When you start writing a new message, you are presented a nice dialog in which you can enter all kinds of information. In the From: line you find a dropdown list. In there you can select the e-mail account you want to use as the sending account.
In the To: / Cc: / Bcc: part, you write the e-mail address of the receiver. You can enter several e-mail addresses, as long as they are separated by commas. An important utility here is the button "Address". When you click that, you see the address book. You can point at an address, click the "To:", "Cc:" or "Bcc:" button, and Sylpheed will place the address in the selected line. The Bcc: may be greyed out. In order to make that accessible, you need to add the Bcc: line to the message. You do that through the Message menu, and checking the Bcc option.
In the Subject line you enter a short descriptive text what your message will be about.
You can move between these lines by pressing TAB (to skip to the next line) and SHIFT-TAB (to go back up). When you press TAB in the last line, you will be moved to the message body.
Did you select a signature file in the settings for the active account, it will be in the message text already. You can now write your message. When you are done, you can click Send, Send later or Draft in the toolbar. Send attempts to send the message immediately. Send later will place it in the Send folder, and Draft will place it in the Draft folder.
To reply to a message, you use the "reply" function. In Sylpheed you activate this through either the "Reply" button, or by pressing Alt-R.
A new message window will be created, with the e-mail address of the recipient and the subject already set for you. Most of the things described in Composing a new message are also applicable to this window.
If you want to have Sylpheed quote the text you are replying to, you need to go into the Configuration menu. There you select Common preferences, go to the Compose tab, and check the "Quote message when replying" box. There you can also edit the quotation mark (the mark put in front of each quoted line in the reply mail) and the reply attribution (which is placed in front of the reply text).
Forwarding a message means: you send the entire message, including headers (Sender address, date sent etc.) on to someone else. For this you press the "Forward" button, or you press Ctrl-F. A new message window is created, containing the text of the selected message. You can then supply the name(s) of the recipient(s) of the message as in composing a new message, perhaps add a few lines of text to the message and send it on its way.