Can you "forward" the email - NOT as an attachment May be your best bet
based on the information in the link provided in your error message . . . . http://pobox.com/~djb/docs/smtplf.html . . . . I don't think this is
something "you" can fix. Are you using a work email? Are the OS and mail
system software current?
That error can be caused by interference from an antivirus program.
Which antivirus are you running?
As a minimum, email scanning in the antivirus should be turned off,
although that may not be sufficient to eliminate all bad effects.
In a worst case scenario, your antivirus may need to be uninstalled.