Whether reply or just an email from the other side, you have to make sure:
1. That your exchange sends mails properly without errors
2. your exchange receives emails properly as well
3. does this error appear on specific recipients or always?
4. Is your exchange designed on Hub transport and Edge Transport?
5.How about sending emails within the organization?
6. Are you facing this problem on web outlook, or outlook or others?
7. Always make a backup for emails, so it won’t be a big issue
the meaning is to isolate the problem