mail: unsupport smtp.verifycert (BC) smtp.verifycert was accidentally broken by cca59ef27e60. And, I believe the "loose" value has been broken for longer than that. The current code refuses to talk to a remote server unless the CA is trusted or the fingerprint is validated. In other words, we lost the ability for smtp.verifycert to lower/disable security. There are special considerations for smtp.verifycert in sslutil.validatesocket() (the "strict" argument). This violates the direction sslutil is evolving towards, which has all security options determined at wrapsocket() time and a unified code path and configs for determining security options. Since smtp.verifycert is broken and since we'll soon have new security defaults and new mechanisms for controlling host security, this patch formally deprecates smtp.verifycert. With this patch, the socket security code in mail.py now effectively mirrors code in url.py and other places we're doing socket security. For the record, removing smtp.verifycert because it was accidentally broken is a poor excuse to remove it. However, I would have done this anyway because smtp.verifycert is a one-off likely used by few people (users of the patchbomb extension) and I don't think the existence of this seldom-used one-off in security code can be justified, especially when you consider that better mechanisms are right around the corner.

--- a/mercurial/mail.py	Tue Apr 05 07:30:01 2016 +0200
+++ b/mercurial/mail.py	Sat Jun 04 11:13:28 2016 -0700
@@ -106,13 +106,6 @@
     mailhost = ui.config('smtp', 'host')
     if not mailhost:
         raise error.Abort(_('smtp.host not configured - cannot send mail'))
-    verifycert = ui.config('smtp', 'verifycert', 'strict')
-    if verifycert not in ['strict', 'loose']:
-        if util.parsebool(verifycert) is not False:
-            raise error.Abort(_('invalid smtp.verifycert configuration: %s')
-                             % (verifycert))
-        verifycert = False
-
     if smtps:
         ui.note(_('(using smtps)\n'))
         s = SMTPS(ui, local_hostname=local_hostname, host=mailhost)
@@ -133,9 +126,9 @@
         s.ehlo()
         s.starttls()
         s.ehlo()
-    if (starttls or smtps) and verifycert:
+    if starttls or smtps:
         ui.note(_('(verifying remote certificate)\n'))
-        sslutil.validatesocket(s.sock, verifycert == 'strict')
+        sslutil.validatesocket(s.sock)
     username = ui.config('smtp', 'username')
     password = ui.config('smtp', 'password')
     if username and not password: