Gmail seems to have a problem with mailing lists. Heck knows why, it
ought to know how they work, by now. The problem being that when you
post through through the list, *it* is not an authorised sender for you
(you're still the email's from address). Your ISP is an authorised
sender for you.
I use gmail for lists. For a few list, gmail flags some messages as SPAM
unitil I have clicked on "not spam" a few times. The resulting discussions
mentioned issues with DMARC (which can use SPF and/or DKIM) -- yet
another part of the overhead running a mail list.