Some emailing accounts can have the option to send email 'On-behalf-of...' which would use a different server to route the emails through. This would then involve multiple emails in reply. Otherwise a different example could be if a php application sends emails through a server - you can specify a return address or addresses (sometimes you can even spoof a reply address).
Obviously the best habit would be to have your clientele to post only one reply address for locating emailing transactions and other debugging tasks (if required).
" Once the transmission channel is established and initial handshaking
completed, the SMTP client normally initiates a mail transaction.
Such a transaction consists of a series of commands to specify the
originator and destination of the mail and transmission of the
message content (including any headers or other structure) itself.
When the same message is sent to multiple recipients, this protocol
encourages the transmission of only one copy of the data for all
recipients at the same destination (or intermediate relay) host."
Standard for ARPA Internet Text Messages
The "From" field must contain machine-usable addresses (addr-
spec). Multiple addresses may be specified, but named-lists
(groups) may not.