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Damn, wasted about 7 hours on this one. I so wish I didn't have to run a mail server. Anyway, the issue was that Qmail was not allowing outgoing messages with 15 or more recipients. Up through 14 would be fine, but then at 15 it would reject all messages (including the first 14) with this error: sorry, reached maximum number of recipients for one session (#5.7.1 - chkuser). Error 5571
So obviously Chkuser is the problem. But how to change this? I took a bunch of wrong turns trying to create a /var/qmail/control/maxrcpt file, but it turns out my Qmailtoaster installation is not compiled to understand the maxrcpt control file. So no luck there.
Hours of googling later I finally found the mailing list question that put me on the right track: I needed to edit tcp.smtp (which I was told would be at /etc/tcp.smtp but of course nothing is easy and mine was instead at /etc/tcprules.d/tcp.smtp). So I changed CHKUSER_RCPTLIMIT="15" to CHKUSER_RCPTLIMIT="150" in tcp.smtp. And then ran qmailctl cdb to reload the new rules (I guess this makes the tcp.smtp.cdb file out of the tcp.smtp file that just got edited.)
And now it works. Fun fun fun.