I uploaded my master key's public part to the keys.openpgp.org-keyserver. It has 4 UIDs set up (4 different e-mail addresses) and each of them are correct and valid e-mail addresses.
The keyserver sent a confirmation mail to all of the e-mail addresses specified in my UIDs, but I did not receive a confirmation link on my outlook.de address. Of course, I checked junk folders etc., but there is simply nothing.
Since all other e-mail addresses could be successfully reached and thus verificated, I am now left with a certificate missing one entry - and this missing entry will likely lead to the removal of the certificate after 14 days.
Why did this happen? Is it because I altered my primary UID? Alphabetically sorted, the outlook.de-address would be the first one, but I manually overwrote this via gpg command line before submitting the public cert to the keyserver.