Hi there I get the following server reply while sending an email to any mailbox under telefonica.net (biggest ISP in Spain)

This is the mail system at host zimbra.mydomain.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<xxxxxxxx@telefonica.net>: host tnetmx.telefonica.net[] said:
552 5.2.0 Invalid 7bit DATA (in reply to end of DATA command)
I get no errors with other mail providers.

May you please help me with this error message?

Thanks and best regards.