keys.openpgp.org not sending confirmation email

[Binarus]

Dear all,

at first, thank you very much for providing privacy and safety for such
a long time free of charge to us!

Second, the following question could be slightly off-topic because it is
partly about the behavior of a key server which clearly is not part of
the GPG software. However, I think I have done something bad to my
Gpg4Win configuration, so I hope I don't get flamed.

I have used the Thunderbird / Enigmail / Gpg4Win troika for quite a
while without any issue. Yesterday, I had to reinstall, and while doing
so, upgraded to the newest versions of that packages, and while I was at
it, revoked my old (1024-bit) keys and generated new (4096-bit) ones
(using Enigmail's key management).

So I got four new key pairs, each of them associated with exactly one
email address. I uploaded the four public keys, again using Enigmail's
key management, to Enigmail's default key server, keys.openpgp.org.
Enigmail reported success each time.

I got confirmation emails for three of that four keys, but it seems that
the key server isn't in the mood to send a confirmation email for the
fourth. I have uploaded that one multiple times since then (again via
Enigmail's key management), each time getting a success message, but
still getting no confirmation email.

I am absolutely sure that this isn't some spam-filter-related issue or
similar. I had a look onto what the MX for the email address in question
was logging when uploading the key. I did this several times, and each
time I watched the logs at least for five minutes after having uploaded
the key. No email message from the key server arrived.

I have carefully checked multiple times that there is no typo in the
email address which is associated with the key in question.

And as expected, when searching for the first three keys on that key
server, they are found, while the fourth is not found.

Now I fear I have done something silly to the Gpg4Win settings (although
I haven't changed its configuration by intention), and I hope that
somebody could explain what could have gone wrong here.

I have quite a good idea of how PGP encryption works in general, but I
never (until now) have used GPG's command line interface (just hadn't to
do so because Enigmail did its job quite good), so it would be nice if
the issue could be explained in simple words :-)

Thank you very much in advance,

Binarus

P.S. If it would help, I wouldn't have a problem with publishing the
email address and the public key in question here.