How can I decode a base64 mail message?

The "Modified Base64" alphabet consists of the MIME base64 alphabet, but does not use the "=" padding character. UTF-7 is intended for use in mail headers (defined in RFC 2047), and the "=" character is reserved in that context as the escape character for "quoted-printable" encoding. However, UTF-7 requires no output padding symbol, as the number of input octets is always a multiple of two, and the potential two octets of zero-bit padding at the end of the encoded data are recognizable as an invalid Unicode character.