The send tag should not be required to be the last tag inside the mail tag. The mail tag should send the mail when the closing tag is reached. I understand you can do something like this <mailer:send>error message</mailer:send> in which case you could use the send tag. It just shouldn't be required. Thanks.
This changes the current behaviour of the taglib. We will look at this again when the next major revision of the taglib is developed.