Hm. Maybe it needs a &from and &fromname set, although it should default to your site’s default email address (set in the Configuration, User tab), looking over the code the &automessage parameter gets loaded into a variable $autotext in the snippet code
'autotext' => isset($automessage)? $automessage:""
which gets checked for document ID or chunk or snippet
if($autotext) $autotext = (($tmp=efLoadTemplate($autotext))!==false )?$tmp:$_lang['ef_no_doc'] . " '$autotext'";
...
$tpl = ( $doc=$modx->getChunk($key) )? $doc : false;
...
Then it gets merged for any placeholders, and mailed
$isHtml = ($sendAsText==1 || strstr($sendAsText,'autotext'))?false:true;
if ($autotext && $fields['email']!='') {
$autotext = formMerge($autotext,$fields);
$mail = new PHPMailer();
$mail->IsMail();
$mail->CharSet = $modx->config['modx_charset'];
$mail->IsHTML($isHtml);
$mail->From = ($autosender)? $autosender:$from;
$mail->FromName = ($autoSenderName)?$autoSenderName:$fromname;
$mail->Subject = $subject;
$mail->Body = $autotext;
AddAddressToMailer($mail,"to",$fields['email']);
if(!$mail->send()) return 'AutoText: ' . $_lang['ef_mail_error'] . $mail->ErrorInfo;