StringIndexOutOfBoundsException when parsing malformed message headers

[octo]

Note: This problem can only occur with POP, since IMAP parses headers server-side.

When parsing a message that is malformed, so that the blank line between the headers and the body is missing, the above-mentioned exception is thrown.

The actual issue is that StringParser.parseMailHeaders() is not tolerant of cases where a seemingly-normal header line does not contain a ":" and a " ".