Text and HTML message bodies are encoded with the quotedprintable mechanism by default. All other attachments are encoded via base64 if no other encoding is given in the addAttachment() call or assigned to the MIME part object later. 7Bit and 8Bit encoding currently only pass on the binary content data.

Zend_Mail_Transport_Smtp encodes lines starting with one dot or two dots so that the mail does not violate the SMTP protocol.