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.