My office and I are experiencing a problem where emails appear to be blank in Apple Mail.
What I can diagnose is that on the GMail server, the messages are Multi-part MIME with a text/plain section and a text/html section. Using the GMail web interface, I can see the contents of the message, and in the Show Original, I can see the plain and html sections fully populated.
On the desktop Apple Mail, the html section is stripped of everything except the MIME Boundary identifiers and contents identifiers:
--Apple-Mail=_XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXX
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html; charset=us-ascii
--Apple-Mail=_XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX--
If Avast stripping out everything between <html> and </html> by accident?
macOS: 10.13.6 (and others)
Apple Mail: Version 11.5 (3445.9.7) (and others)
Avast Security 14.6 (0ee7f50937ff)