My user sent an email which lead to the case creation. In the case I found some attachments are missing so I want to send an email to same mail thread. How can I achieve this. And after user reply back with the attachment or missed attachment the new attachment needs tobe added to the case. Is it feasible?
Hey @MANOHAR7991 , welcome to the community 
!
Yes, this is feasible and PEGA has OOTB support to continue the same email thread, receive follow-up replies, and automatically add missing attachments to the existing case.
When a case is created from an inbound email, PEGA already has the email context (headers, message ID, sender).
-Use OOTB Correspondence / Data-Corr-Email (preferred in App Studio & Constellation).
-Key is to reply from the case instead of sending a fresh email.
-Ensure the following is preserved if your configurations (OOTB behavior):
In-Reply-To / Message-ID headers, or
Case ID in the subject, or
Tracking token in the email body
To add what @VikasT has already mentioned, in many implementations, the thread continuity is maintained by the Email Triage / EmailManager flow rather than the business case alone. So if your application uses a separate triage case and service case pattern, make sure the reply is sent from the thread-tracking entity and not as a detached outbound email. That is what preserves the correlation and allows the reply attachment to land back on the right case.
Also, verify the inbound attachment-processing path. Inbound emails can arrive with the attachment intact, but the case still needs the correct email-to-case processing and attachment linkage for it to show up on the existing case