Some emails don't arrive in outlook the first time.

Many employees complain that when they send an email to an external recipient, the email actually arrives.
When the recipient replies, the answer doesn't arrive.
The employee sends a reminder to the recipient, and the recipient sends him an email with his previous email attachment. This last e-mail reaches the employee.
We've checked the quarantine, spam, ... and the first reply email doesn't appear anywhere.
It doesn't happen all the time, and it doesn't happen to everyone.

Do you have any ideas? Have you had any experience of this?

Thank you in advance for your help.