Self Emails and Account Users Email Not Syncing

Hi All,
I have installed erp v13.35 and added users to the account. I found that when I send email to my colleagues (account user in erp) it sends but does not reach his inbox, but the email is updated in his Contact page. Please help me how to resolve this issue!

check Email Queue list, it’s satus should be sent

All the emails are sent. It is just the receiving issue.
Weirdly though, all outside emails are syncing in his inbox and mine too. It is as if erp does not want to sync inbox of receivers (of same domain emails).