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If you're sending to/from Gmail/Exchange the limit is somewhere around 100 addresses (I believe it's technically the byte-size of the field not strictly the number of addresses).

The actual spec though, AFAIK, has no limit.

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Right. SMTP header lines can't exceed 998 characters, but RFC2822 now allows multi-line headers[1], so there's no limit except one that might be imposed by an MTA (and that would generally only be imposed on sending, not receiving.)

1. https://www.ietf.org/rfc/rfc2822.txt §2.2.3

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