My personal view... that spam filter is rubbish. But I am probably biased. I rarely have an image in my emails and often they go out as plain text because that is all they are. So they would fail two of the three criteria you mention.

As for the form an address may or may not be "encoded". I think you should refer to the relevant RFC which sets the standards. RFC 5321 appears to support the standards requirement that the to address be surrounded by <>

I see all mail generated in my copy of Thunderbird as containing a <> in the To field. So are you talking about incoming mail? If so the "policing" of the syntactical validity of an email is made by The SMTP server the mail is sent through. Thunderbird might include a similar weighted token in it's spam filtering as a part of it's learning database. But it is not up to the mail client to examine the validity of the incoming mail. Only to display it to the best of it's ability.

hi Matt,

Thanks for your swift reply.

I'm refering to outgoing mail. I see mail in my "sent" folder that does indeed only have the "test@test.com" markup instead of the "<test@test.com>" or "John Doe <test@test.com>" markup.

the non brackets situation occurs when i type an email to a new contact person.

If i reply to an email in the inbox, Thunderbird adds the brackets perfectly fine.

So i wonder why the brackets are not included when i type a new email to a new email address (which is not in the contact list)