I'm getting an extra space in the mail subject when the subject has many characters. One such instance is when receiving the password reset email. It ends up being "Subject: Replacement login information for Some User Name at Some Long Site Name ((two spaces here)Secondary Name Here)".
I traced the extra space in the subject to mimemail.module where drupal_html_to_text is run. I then traced the point where the extra space was being introduced and it is in drupal/includes/mail.inc in function _drupal_wrap_mail_line. As the subject is longer than the 77 characters for the wordwrap call, two extra spaces and a newline are added as a string break character. As it is necessary to rewrite any HTML that might happen to end up in the subject (see: #369983: Plain text Email subject shows & as _amp; and " as _quot; ), the drupal_html_to_text call can't be eliminated.
It seems to me from #438058: Remove line feeds in subject that people were experiencing mail client breakage from the subject newlines, even though RFC 2822 says they should be OK. To fix this, the str_replace call was then added to remove the newlines.
An extra space would still exist in long subjects even after the str_replace. A quick fix is to replace in the mimemail.module for mimemail_prepare_message:
$subject = str_replace("\n", '', trim(drupal_html_to_text($subject)));
$subject = str_replace("\n", '', str_replace(" \n", '', trim(drupal_html_to_text($subject))));
I'm not sure if this has any undesirable side-effects, however. Obviously my change could be refactored to look better as well. After the change, my email subjects look like they should without the extra space.