UniversalMailer is a Mail.app plugin that aims at setting the default font for outgoing e-mails and fixing issues with attachments. Enable the plugin, set a default font for outgoing e-mails and you're done: the next e-mail you'll send will have a nice default font and won't show those annoying ATT00001.htm attachments any more. The HTML signature is not strictly needed, since the plugin can automatically choose to send riched text e-mail for you.
Comments