Section 3.2.3 suggests that the local part of an address may be a dot-atom or a quoted string, but not a combination of both as your example shows. Do mail clients accept this address?

There is also section 4.4 describing obs-local-part = word *("." word) so that email address should be valid. Also mail send like

$ sendmail -t
From: me@some.domain
To: foo."x".bar@some.domain
Subject: Test

Test.

does arrive.

Note: While section 4 is obsolete, it must be accepted.

Though these syntactic forms MUST NOT be generated according to the grammar
in section 3, they MUST be accepted and parsed by a conformant receiver.

I read “.” as one literal period character

Well yeah, so the obs-local-part in foo."x".bar should be evaluated like atom "." quoted-string "." atom, which looks valid based on:

obs-local-part  =   word *("." word)
word            =   atom / quoted-string

Am I reading the RFC wrong?

Tbh I’m not sure. What’s your use case? Do you have a need to accept this form of address?

I've tried to put list of valid email addresses [0] through our email validation and some did not pass. So it seemed reasonable to open a bug report about this. If full RFC 5322 compatibility is not a desired goal, then this can be closed.

[0] https://github.com/StoneCypher/testdata-valid-email/blob/master/testdata-valid-email.js