download mail error message

I'm using the download mail plugin and have noticed on two occassions that it has seemingly stopped working so no new messages have been downloaded.

To investigate, I've removed the  '> /dev/null 2>&1' from the end of the crontab command and as a result can see the error to be the following (I've changed the email address and domain name for this post):

Checking mail account: abc@123.com Connecting to POP3 host: mail.123.com Downloading 5 of 34 new messages Downloading message #1 - UID: 1 - Message-Id: <OF76F08218.3AD78194-ON80257B72.0043F25C-80257B72.004474DA@notes.rsun.com>
Unknown subtype (msgno 1): <xmp>stdClass Object (
    [type] => 5
    [encoding] => 3
    [ifsubtype] => 1
    [subtype] => JPEG
    [ifdescription] => 0
    [ifid] => 1
    [id] => <_2_0C45C3140C45C0A8004474D780257B72>
    [bytes] => 24256
    [ifdisposition] => 0
    [ifdparameters] => 0
    [ifparameters] => 0
    [parameters] => stdClass Object
        (
        )

)
</xmp>

This has happened with two seperate emails and removing them from the mailbox then 'frees' the process up.

Do you have any ideas what the cause could be?

Thanks, Paul.