Mail server reports “550 Address unknown”, but check via telnet finds the address

Hello,

I'm trying to send an email through Postfix from mydomain.com to an address I know exists (info@example.org) - the receiving end seems to be running a Sophos product.
Postfix is reporting that the address does not exist.

Excerpt from my mail log

Jun 7 07:01:47 mydomain postfix/smtp[50352]: 010BB40606: to=<info@example.org>, relay=remote.example.org[redacted]:25, delay=1.2, delays=0.37/0/0.37/0.48, dsn=5.0.0, status=bounced (host remote.example.org[redacted] said: 550 Address unknown (in reply to RCPT TO command))

Here is the telnet session (from the server where Postfix runs) showing that the address exists:

# telnet remote.example.org 25
Trying [redacted]...
Connected to remote.example.org.
Escape character is '^]'.
220 sophos.example.local ESMTP ready.
HELO mydomain.com
250 sophos.example.local Hello mydomain.com [redacted]
MAIL FROM: <myaddress@mydomain.com>
250 OK
RCPT TO: <info@example.org>
250 Accepted

To me it looks like my email is rejected by Sophos for some reason. From what limited information I could gather so far, it seems to be related to "recipient verification". 

Could someone help me understand what kind of check the other side is running that keeps my email from being delivered? The email address I'm sending from and the address in the return path do exist (which can be checked by using telnet). 

Thanks!