Undeliverable Mail \ Unknown Exception

I am seeking assistance in identifying the cause of an issue that results in undeliverable email when using cfmail.   On occasion (4 to 12 times in a 24 hour period) an email will be determined to be undeliverable and will be transferred to the /Undelivr directory.  Entries exist in the exception.log, however they are of no help in identifying the issue.  The first three lines of the entry appear as follows:

"Error","scheduler-6","12/03/09","06:04:44",,""
javax.mail.MessagingException:
at com.sun.mail.smtp.SMTPTransport.issueCommand(SMTPTransport.java:879)

Note that there is no detail message in the “javax.mailMessagingException:” line.

A corresponding message is written to the mailsent.log.  It appears as follows:

"12/03/09","06:04:44",,"Moved undelivered mail: Mail19743.cfmail to /usr/apps/coldfusionmx/Mail/Undelivr directory" "Information","scheduler-6"

Finally, the message in the mail.log file appears as:

"Error","scheduler-6","12/03/09","06:04:44",,""

The email messages that are deemed to be undeliverable are successfully delivered when they are re-spooled for delivery.

In an effort to determine if the problem is associated with the SMTP server, the environment was configured to use a different server.  The same problem occurred using both SMPT servers.

A summary of the environment is:

CF MX 6.1 (Java 1.4.2_11) running on Solaris 9

Hot fixes applied:
hf45343_611.jar
hf55681_611.jar
hf56580_611.jar
hf59763_611.jar
hf59993_611.jar
hf64586_611.jar
jrun-hotfix-57510-updater4.jar

Thank you in advance for your assistance.