Was ist nun richtig, resp. was verlangen andere Mailprogramme?
RFC 2822: 3.4. Address Specification
"[...] An address may either be an individual mailbox, or a group of mailboxes. [...]
mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list
address-list = (address *("," address)) / obs-addr-list
Mit anderen Worten: Ein Beistrich / Komma ist richtig.