Failed to deliver message

As of yesterday I'm receiving a series of..."failed to deliver" messages being kicked back from several AOL addresses- with whom I communicate with all the time. As the" failed to....msg." is for more than one email address - I'm curious as to what else might prompt the failure to deliver message. These were only text messages -with no attachments.
...
1. Are these only indicating the receivers email box is full ? and it's just a coincidence that two or three recipients have full mail boxes ?

2. or is there a subtle setting that I/m not aware of that is causing a problem on a "554 transatction failed"

opening error message reads:

subject : Undeliverable mail: email

Failed to deliver to 'xxxx @aol.com'
SMTP module(domain @137.118.16.124:aol.com) reports:
message text rejected by mailin-03.mx.aol.com:
554 TRANSACTION FAILED

Received: from [137.118.16.62] (HELO mx1.av-mx.com)
by relay0.av-mx.com (CommuniGate Pro SMTP 4.2.7)
with SMTP id 264647236 for (xxx-address
Mon, 22 Aug 2005 09:34:58 -0400
Received: (qmail 15285 invoked from network); 22 Aug 2005 13:34:58 -0000
Received: from (address)

Is there anything OI can do on my end that cause a problem?

TOOTS