Text and HTML message bodies are encoded with the quotedprintable mechanism by default. Message headers are also encoded with the quotedprintable mechanism if you do not specify base64 in setHeaderEncoding(). If you use language that is not Roman letters-based, the base64 would be more suitable. 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.

Header Encoding, especially the encoding of the subject, is a tricky topic. Zend_Mime currently implements its own algorithm to encode quoted printable headers according to RFC-2045. This due to the problems of iconv_mime_encode() and mb_encode_mimeheader() with regards to certain charsets. This algorithm only breaks the header at spaces, which might lead to headers that far exceed the suggested length of 76 chars. For this cases it is suggested to switch to BASE64 header encoding same as the following example describes:

// By default Zend_Mime::ENCODING_QUOTEDPRINTABLE
$mail = new Zend_Mail('KOI8-R');

// Reset to Base64 Encoding because Russian expressed in KOI8-R is
// different from Roman letters-based languages greatly.
$mail->setHeaderEncoding(Zend_Mime::ENCODING_BASE64);

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