Hi fariha,
As this previous post said: marketing emails are sent by different back end engine compared with emails in activity, so they will not be synced to emails in activity and we can't see reply from contacts in customer journey in CRM.
However, as we can will still receive/watch contacts reply email for marketing email in Outlook mailbox, you can still track contacts reply email for marketing email by adding/enabling Dynamics 365 App for Outlook for your marketing emails sending mailbox.(As you add it in Dynamics 365 CE, in Settings > System > Dynamics 365 App for Outlook),
which means all of these emails will all be synced as activity emails, include original marketing email.
If you use your own email server, it may not works.
