Email Woes

[FrankT]

Hi!

I can send email just fine with both Horde and Squirrelmail, but when I try to reply to the emails I sent (from my work ISP and email address), I get this error when attempting to reply:

<<< 550-"The recipient cannot be verified. Please check all recipients of this <<< 550 message to verify they are valid." 550 5.1.1 <user@franktuttle.com>... User unknown <<< 503 valid RCPT command must precede DATA

I must have skipped a step somewhere, but I can't figure out what I've done wrong.

Any ideas? Thanks!