It may be, but your mail gateway at (logic) says NO.  I just tried the
same address (all upper and all lower case).  I then tried the explicit
MX hosts:


jtsa.edu        MX      5       logic.jtsa.edu
jtsa.edu        MX      10      sem.jtsa.edu

and LOGIC bounced, but SEM at least (so far) accepted the test message.
Fix your LOGIC ;-)


PSUVM.PSU.EDU unable to deliver following mail to recipient(s):
    <[log in to unmask]>
PSUVM.PSU.EDU received negative reply:
553 5.1.1 unknown or illegal user: [log in to unmask]

           ** Text of Mail follows **
Received: from PSUVM.PSU.EDU by PSUVM.PSU.EDU (IBM VM SMTP V2R2)
   with BSMTP id 2530; Tue, 28 Apr 98 15:54:18 EDT
Received: from PSUVM.PSU.EDU (NJE origin PMW1@PSUVM) by PSUVM.PSU.EDU (LMail
 V1.2c/1.8c) with RFC822 id 5813; Tue, 28 Apr 1998 15:54:18 -0400
Date:    Tue, 28 Apr 98 15:54 EDT
From:    "Peter M. Weiss" <[log in to unmask]>
To:      [log in to unmask]

--


At 14:30 4/28/98 -0400, BRIAN MC NAMARA said:
|below is the message bounce that keeps getting returned even though the
|address, [log in to unmask] is up and live and active.
|
|I hope you have some ideas.
|
|thanks.
|
|Brian
|-----Original Message-----
|From:   Mail Delivery Subsystem [SMTP:[log in to unmask]]
|Sent:   Tue, April 28, 1998 6:34 PM
|To:     [log in to unmask]
|Subject:        Returned mail: User unknown
|
|  The original message was received at Tue, 28 Apr 1998 18:33:45 +0300
(IDT) from ts006p6.jlm.netvision.net.il [194.90.120.116]
|----- The following addresses had permanent fatal errors -----
|<[log in to unmask]>
|----- Transcript of session follows -----
|.. while talking to logic.jtsa.edu.:
|>>> RCPT To:<[log in to unmask]>
|<<< 553 5.1.1 unknown or illegal user: [log in to unmask]
|550 <[log in to unmask]>... User unknown
|
|
|
|
|
|
|Attachment Converted: "C:\IACCESS\EUDORA\Attach\ATT00001.att"
|Date: Tue, 28 Apr 1998 18:36:02 +0300
|From: Dvora Bitcover <[log in to unmask]>
|Subject: test
|To: "[log in to unmask]" <[log in to unmask]>
|MIME-version: 1.0
|Content-type: text/plain; charset=us-ascii
|Content-transfer-encoding: 7bit
|
|This is a test - do not respond
|
|