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Some people are saying the emails were in the message body, not CC field.

What's the limit for numbers of addresses in a CC field? Because this is several thousand addresses.

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.

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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