Have you turned allow_envelope_sender “On”, and uncommented default_envelope_sender and force_default_envelope_sender?
When sending through Gmail, gmail will validate that the user OJS is claiming to be the sender. This means that if a user with an email address at yahoo.com sends a message via OJS, gmail will flag it as spoofed, and the recipient is likely to see it flagged as spam. (A technical explanation of SPF and DMARC is here.)
If a message is unable to be sent at all, you should see a warning flash in the notification area within OJS, and an error will be logged in the PHP error log.