Any line following a blank line and beginning with the four characters "From" marks the start of a new message. The BSD way of handling such lines in the body of a message was to prefix them with ">", which is a non-reversible workaround and is in the uuencode target charset.

Prefixing every line in the message body with a tab character would allow reversing and not interfere with uuencode/base64&c.