501 5.1.3 Bad recipient address syntax

I did remove most personal information, just wondering what the cause/fix would be. This is the only teacher that has mentioned this problem. There where about 15 recipients that either had a Yahoo address or a 'Y' in their names. It turns the 'Y' into a '?'. Any idea?

The message that you sent was undeliverable to the following:

User@?ahoo.com (501 5.1.3 Bad recipient address syntax)

patt?-user@?ahoo.com (501 5.1.3 Bad recipient address syntax)

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Subject: Subject
GroupWise Message Id: 55D3491F.BEC:87:14877
Message log tag: 13243462
Number of send attempts: 1
Time of initial send attempt: 08-18-15 10:03:06

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Received: from Lawson-MTA by mail.lawson.k12.mo.us
with Novell_GroupWise; Tue, 18 Aug 2015 10:03:06 -0500
Message-Id: <55D302CF02000057000456DE@mail.lawson.k12.mo.us>
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Date: Tue, 18 Aug 2015 10:02:55 -0500
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