Für alles gibt es eine RFC, in Falle Deiner Anfrage RFC 2368:
ftp://ftp.isi.edu/in-notes/rfc2368.txt
Zitat:
The mailto URL is primarily intended for generation of short text messages that are actually the content of automatic processing (such as "subscribe" messages for mailing lists), not general MIME bodies.
Within mailto URLs, the characters "?", "=", "&" are reserved.
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=> Du kannst "&" nicht im Body verwenden.
Sorry.