Trouble sending file from Calibre to Kindle

I converted my files to mobi and trying to send to my kindle. It has worked in the past but is no longer working. This is the error message I receive:

Sending failed...

Traceback (most recent call last):
File "site-packages/calibre/gui2/email.py", line 43, in run
File "site-packages/calibre/gui2/email.py", line 124, in sendmail
File "site-packages/calibre/utils/smtp.py", line 116, in sendmail
File "site-packages/calibre/utils/smtp.py", line 108, in sendmail_direct
IOError: Failed to send mail: IOError(32, 'Broken pipe')

Job: "Email Dancing Girls and Other Stories to ZZZZZ@free.kindle.com" failed with error:
Traceback (most recent call last):
File "site-packages/calibre/gui2/threaded_jobs.py", line 83, in start_work
File "site-packages/calibre/gui2/email.py", line 98, in __call__
IOError: Failed to send mail: IOError(32, 'Broken pipe')

Called with args: (u'/var/folders/nj/sk2rpyc94gvfr8vm2wq0_61c0000gn/T/calibre_2.10.0_tmp_4kUXnv/H4fpo8caltmpfmt.mobi', u'Dancing Girls and Other Stories - Margaret Atwood.mobi', u'ZZZZ@free.kindle.com', u'E-book: Dancing Girls and Other Stories', u'Attached, you will find the e-book\n\nDancing Girls and Other Stories\n\tby Margaret Atwood\n\nin the MOBI format.') {u'log': , u'abort': , u'notifications': }

Any ideas how to help? THANKS!