1 - When a subject has quotes '"', ENVELOPE will not quote the subject. Example:
From: foobar.com Subject: Here's your "gift" To: foobar.com
ENVELOPE will contain "Here's your "gift""
Because messages ENVELOPE can be included in BODYSTRUCTURE, BODYSTRUCTURE can neither be used.
2 - Yahoo allows only one exclusive connection. When multiple connections are used, FETCH will fail silently.
3 - Yahoo connection has to be closed, then reopened to get new emails.