The Composer window is used to write new messages. It can be invoked from the Message menu or from the document icon on the Mail Reader window button bar. If you have not set the location of your signature file and you have Use a signature from file checked in the Composer tab of the Configure dialog, you will be prompted for the location of your signature file before the Composer window opens.
To write your message, fill in the appropriate fields in the Composer window. There are a variety of shortcuts to help you with writing your messages. The ellipsis buttons next to the To:, Cc:, and Bcc: fields will call up the address book so that you can select addresses from there. If you already started typing an address you can also press Ctrl+T to get a list of possible completions. Whenever you want to add more than one recipient in one of the fields, use a comma to separate each address from the next one. You may need to specify fully qualified addresses (i.e. user@domain.com) even for local users.
You can use the View menu to set up the header fields offered to you. When you are finished with your message, hit the Send button (the envelope) to queue or send the message. If you don't want to send your message yet, select Message->Save in drafts folder.
You can attach files to your message by using one of the methods below:
Push the paper clip button and select the file you wish to attach.
Drag a file from the desktop or another directory into the Composer window.
Selecting one of the options in the Attach menu.
The Message Part Properties dialog will appear to prompt you for information about your attachment. The MIME-type and file name fields are automatically filled in with the appropriate values (Sometimes the MIME-type value may be wrong. You can then type in any content MIME-type or choose from the list of common MIME-types). You should then select an encoding method for your file from the list of encoding options (normally, the default value works fine).
Once a file is attached to your message, it appears in the Attachments pane at the bottom of the Composer window. You may remove, save, or get information about an attachment by selecting its name in the Attachments pane and selecting Remove, Save..., or Properties... from the Attach menu.
PGP/GnuPG keys may also be attached to your message by using the appropriate options in the Attach menu. PGP key attachments are handled like file attachments.
To check the spelling of your message, select Edit->Spelling.... KMail uses KSpell to check spelling, which is the KDE frontend to the the ispell or aspell spelling checker. Note that you may need to configure the spellchecker using Settings->Spellchecker....