process-send-string broken between 21.2.20 and 21.2.22/23 under NT

Hi, On my system, 21.2.22/23 fails while sending base64-encoded data via smtp. Using smtpmail-debug-info, I see that it starts sending lines of base64 encoded data, but after about 100-150 lines (the exact number varies) process-send-string just hangs. C-g cancels it successfully but the message does not get sent of course. This is 100% consistent, at least with a certain zipped file (.zip is around 115k) which I try to send as a mime attachment from gnus. Same gnus, same lisp code (in particular, the same mail-lib package) sends the same file successfully under 21.2.19 and 21.2.20. This is under NT 4.0 sp5, native win32 build. Any help/ideas would be greatly appreciated. Thanks. -- Cheers, -Dima.