Found this in my Gmail outbox and was like WTF… Apparently, messages like these are generated when you unsubscribe from some mailing lists (as a unique identifier, I guess).
NUpqVnVsSXZ5cDhjQlByTlVOTFhDNkVwbmZ5ejdiS2xmV1h3UHZyRnpDanRtb3Vncit2WGdONGxoRlBh