Adding an attachment removes the line feed between headers and body.

[hbog]

Attachments create a A: pseudo header, but this removes the (RFC defined) newline between headers and body. This has no impact on sending the mail because the python email package handles the absence of the newline silently (MissingHeaderBodySeparatorDefect). However when adding multiple attachments the A:-pseudo header will be added after the first empty line, which will now be in the body of the message.

For example

• Before adding the attachment

From:
To:
Subject: 

Dear Addressee

Message ...

• After adding first attachment, the newline between headers and body is removed

From:
To:
Subject: subject
A: attachment_1
Dear Addressee

Message ...

• The second attachment pseudo header lands in the body

From:
To:
Subject: subject
A: attachment_1
Dear Addressee
A: attachment_2
Message ...