My Gnus now refuses to send long emails

[Joe Fineman]

I use Gnus 5.13 under Cygwin 1.5.25 under Windows XP.  For the last few
days it has refused to send any email longer than about 2500
characters, whether in the text proper or in an attachment.  Sometimes
the message is

  smtpmail-send-command: writing to process: no error, SMTP

and sometimes 

  Process SMTP not running.

My Emacs 24.3 tells me that smtpmail-send-command is not documented.
What is going on?

Note:  I have repeatedly tried to post this query on gnu.emacs.gnus.
Gnus goes through the motions, but the posting has never appeared.  I
have had not difficulty posting on other newsgroups, so I am trying this
one.
-- 
---  Joe Fineman    joe_f@verizon.net

||:  Cave canem.  (Beware -- I might sing.)  :||