There certainly is a difference in how mail servers behave in that case. Ideally the receiving mail server will reject the
RCPT command, in which case the mail content will never be delivered to the target server in the first place.
In a much less ideal scenario (which is unfortunately often seen in practice anyway), the mail server will accept the
RCPT command even though the target address does not exist. If the server proceed to accept the incoming mail, it now becomes the responsibility of the receiving server to generate an error message.
This is a problem, because some of the mails will be spam mails with a forged sender address. By accepting and bouncing, the mail server in question is causing spam mails to end up in other people's mail box.
It is easy to test a specific address yourself. You just need to use the telnet command and issue the following SMTP commands
QUIT with the proper arguments. By following the
RCPT command by
QUIT rather than
DATA, you will not actually send a mail to the server.
Here is what it could look like when testing the address
telnet mx.example.com 25
Connected to mx.example.com.
Escape character is '^]'.
250 Bounce OK - Please include Message-ID from original message
550 No such user
221 Closing connection
Connection closed by foreign host.