When the composer is set to HTML email by default, and when sending email to a
recipient that is specified to not want HTML mail, when pressing "send"
evolution pops up a warning dialog offering to send the message any or to cancel
the send. This feature is very useful because a lot of people prefer HTML mail
where one can use basic text formatting features, but there are also recipients
that should always receive text only email (such as mailing lists). 

Mozilla Thunderbird has a nice feature where in the case above the warning
dialog offers the option of immediately sending the email as text only - thereby
changing the format of the mail to text only and sending it. Evolution only
offers to cancel the send, change the format by hand and then sending again. I
think it would be great if Evolution could also offer to do this automatically.