Mail Server for "my domain" unreachable for too long

We have a very important client that frequently needs to send us email.  He, however, continues to receive error messages when sending them.  He is the only person I know of having these problems and I don't know why.  Any help would be appreciated.  The error message is below...

-----Original Message-----
From: MAILER-DAEMON@yahoo.com [mailto:MAILER-DAEMON@yahoo.com]
Sent: Saturday, November 16, 2013 12:30 PM
To: client@swbell.net
Subject: Failure Notice

Sorry, we were unable to deliver your message to the following address.

<myboss@my domain.com>:
Mail server for "mydomain.com" unreachable for too long

--- Below this line is a copy of the message.

Received: from [216.39.60.169] by nm4.access.bullet.mail.gq1.yahoo.com with NNFMP; 15 Nov 2013 18:29:09 -0000
Received: from [98.138.104.97] by tm5.access.bullet.mail.gq1.yahoo.com with NNFMP; 15 Nov 2013 18:29:08 -0000
Received: from [127.0.0.1] by smtp117.sbc.mail.ne1.yahoo.com with NNFMP; 15 Nov 2013 18:29:08 -0000
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=swbell.net; s=s1024; t=1384540148; bh=oULWd5Oc04POxe6dC80x8Zojt5kICIticns5nqOJ3Y8=;
h=X-Yahoo-Newman-Id:X-Yahoo-Newman-Property:X-YMail-OSG:X-Yahoo-SMTP:X-Rocke
t-Received:From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type:X-Ma
iler:Thread-Index:X-MimeOLE;
b=fPp+YXY/oNULrfle6fXpUiNEe+yMuvqVLCu91CzVwWUaNHEtGOL3eoXWyfnM/MzGn4PMsEiyZu
EoieCBW+VAWsDQLn2dbqGcllTg7zUSJDWYPR5B4TUBMx8q9NmCp1kkfZXPOZ8738QCVgJ9WG
EoieCBW+wWZF
Fxq944MFbqf2uHEJTYaPc=
X-Yahoo-Newman-Id: 727170.31783.bm@smtp117.sbc.mail.ne1.yahoo.com
X-Yahoo-Newman-Property: ymail-3
X-YMail-OSG: r5uAZ08VM1mcfAOBsL.X1foq2qI4racZFOc_fLR3_E51i2C
 RezLWhQAXmpVF9_28NhPzWxFEDba7MDTiEZdPBk2eytye1Mm8aKX1Bn2Df4e
 p7aROqsooDbiRr4uNRnD4qbF9syzeQaWG70EKnV7EbkMJhTQJrgqIoQbWBIs
 9BW4xVEaXtMuDBh9pisu0GT.M7THKTEyKDangYJHxE_dwbylz54AP6kNRpS2
 Zqjgk7rxQvumhRMXFxLj9I4RMNwJ0KKPK6D8tglll5hHuMdDDZTowZt4ScNQ
 iyvkgGCE4hVGfD6Tk7jIqeiHoTLAedvRV5FU3ChEj0NQz8GJ9xccyhtrcZBI
 nwRSp3iZOvXkClSuV9VWdDe4dVW8E_7Ik_eZIyFDPwiCJNmBsjmiQ30qwo61
 qnXA3FZ1F732bk_ISe3Ip7H82AqlBDnRsd3wjIAIJhgRD6H1MgEt4E8QHBRF
 kdYq12ht2RXRen.xJFrDxlWgVkXTsSR2TNgCTIFWknSkfES4MTIce7vG90Zd
 hz.LOzIYp3gHvYkTzjgh4FTxyGUgWfpmQuIBn90fV_uoiXPBE.E1AzqbvsCR
 ggs8bQltIENJfAaKQ_YHmfZzF3h_BpgeMmMVkCsa9AfLnDvVPfadD3UTC6zo
 EPyXY164rbQImcwgyUbeCUhw-
X-Yahoo-SMTP: 9w_Wy92swBAx38f1eFCm0bGxEm7cONNyWzHXDNqE2Q--
X-Rocket-Received: from PPosner7lt (pposner@72.179.38.205 with )
        by smtp117.sbc.mail.ne1.yahoo.com with SMTP; 15 Nov 2013 18:29:08
+0000 UTC