[exim] sender verify at verizon.net (sigh)

I run a small mail server (exim4 on Debian) and on that server hosts
several email lists. One of them includes a member at verizon.net,
and his address recently started failing. He changed his address to
verizon.net in mid-December and all worked well until last Friday.

At that point, all email to him (either through the list, or directly)
cannot get through.

The message would be actively rejected thus:

2005-02-21 09:10:53 1D3H4F-00089S-LH == bcebsg@??? <gregorians@???> R=dnslookup T=remote_smtp defer (0): SMTP error from remote mailer after MAIL FROM:<gregorians-outlist-owner@???> SIZE=2524: host relay.verizon.net [206.46.232.11]: 450 Requested mail action not taken-Try later:sv6.verizon.net

And immediately before (coincidence? not!) would be the following
curious log entry:

2005-02-21 09:10:53 SMTP protocol violation: synchronization error (input sent without waiting for greeting): rejected connection from H=vms142pub.verizon.net [206.46.252.142]

Now I'm inclined to blame it on the apparent use of VMS as an
operating system by Verizon. Maybe not.

After more thought, I realize that Verizon is using broken sender
verification, sending a fake message back to the sender of the message
(in this case, gregorians-outlist-owner@???) and the fake
message checkback is not following SMTP synchronization rules!
Idiots. Ok, so I go back to my config and I turn off
smtp_enforce_sync, thinking this should get the checkback to succeed.
Later checking on email lists shows that Verizon does have this bug.
But it's unclear why mail to my correspondent worked fine for two
months before now.

But making this change doesn't help. It still fails with apparently
the same error, though I of course no longer get the synchronization
errors:

2005-02-21 12:38:10 1D3H4F-00089S-LH == bcebsg@??? <gregorians@???> R=dnslookup T=remote_smtp defer (0): SMTP error from remote mailer after MAIL FROM:<gregorians-outlist-owner@???> SIZE=2524: host relay.verizon.net [206.46.232.11]: 450 Requested mail action not taken-Try later:sv3.verizon.net

Sometimes the error is different and the message simply times out:

2005-02-21 11:47:31 1D2yaG-0005rh-E7 == bcebsg@??? <gregorians@???> R=dnslookup T=remote_smtp defer (110): Connection timed out: SMTP timeout while connected to relay.verizon.net [206.46.232.11] after MAIL FROM:<gregorians-outlist-owner@???> SIZE=4682

I would like to see what's happening on the checkback transaction, but
mail transactions that disconnect after the RCPT succeeds don't seem
to be logged.

The IP address of my mail server (becket.becket.net) doesn't seem to
be on any block lists; my ISP is fine with what I'm doing; my DNS is
all happy; and reading mailing list archives suggests that this
happens every now and then. Is there anything I should try other than
just telling my correspondent that as long as he uses Verizon and they
don't fix it, he can't get mail through my server?

Thomas