When replying to an email sent from Netsuite, the reply to email address looks similar to firstname.lastname@example.org or transactions.11223344.123_456_78.dd9c089xyz@AAAA.email.netsuite.com Below is a description on how this email address is formed.
The first part of the string tells you if the email came from a case record / transaction or simply s direct email sent from Netsuite UI. It would show cases for support case emails, transactions for transaction emails and message for emails sent thru netsuite
AAAAAAA = NetSuite Account ID
BBBB = Internal Id value of the case
CCCC = Internal Id value of the last message on the case
DDDD = Internal Id value of the Company/Customer on the case
dd9c089xyz = A random set of characters generated by NetSuite
Note: The string is random and generated by the system