The "MAIL FROM" command argument must always be enclosed in angle-brackets. Some mail servers (e.g. gmail, aol) would reject messages not delivered in this manner, e.g.:
mx-aol.mail.gm0.yahoodns.net replied with:
"501 Syntax error in parameters or arguments tnmpmscs"
instead of the expected reply:
"250 ..."
gmail-smtp-in.l.google.com replied with:
"555 5.5.2 Syntax error. o6si11103060plk.317 - gsmtp"
instead of the expected reply:
"250 ..."