Trouble sending emails to users on same domain

I recently experienced a strange problem. I tried sending an email to another user on our domain. He never received the email in his inbox. Instead it got sent to his junk mail folder. I found it by logging onto We have also noticed that sending emails to other users on our domain can sometimes take longer to receive than messages that come from outside our domain.

I have whitelisted in the junkmail settings. Is there anything else I can do to prevent this issue from re-occurring?

You can whitelist a domain for the whole domain.

I’ve done that already. I was surprised that whitelisting our domain would even be needed.