In August I received an email from Skype thanking me for registering an account. But I already had an account, I didn't register this one. After comparing the new account name with part of my email, I came to the conclusion that someone mistyped their email address, and registered an account on my address. I contacted their live support, here's the conversation:

George A: Hello! Welcome to Skype Live Support! My name is George. How
may I help you?

me: Recently I have received an email welcoming me to Skype (not
phishing, I verified). The problem is that I didn't create the account
mentioned in the email. The account name was "[NEW SKYPE ACCOUNT]" and
my email is [MY EMAIL 1], so I think that user mistyped his email
address, and then Skype sent a welcome message to me. Doesn't skype
verifies email addresses before sending a welcome message?

George A: I understand that you are concerned about your email address
being used to setup a Skype account, I'll be happy to help you with
that. May I please have your Skype Name?

me: [MY SKYPE ACCOUNT]

George A: I would also need the email address, please.

me: [MY EMAIL 1]. let me check that this address in on my Skype
account... ok, my email on file in Skype is [MY EMAIL 2]. and a few
other too, all mine :)

George A: Well, I see that there is only Skype Name registered under
that email address, the Skype Name is [NEW SKYPE ACCOUNT]

me: Yes, for my account ([MY SKYPE ACCOUNT]) the primary email is [MY
EMAIL 2], but other emails on profile are [MY EMAIL 1], [MY EMAIL 2],
[MY EMAIL 3].

George A: May I please ask you to confirm which Skype Name that you do
not authorize?

me: Does Skype sends verification message before assigning the email
to account? The Skype name which I didn't create is [NEW SKYPE
ACCOUNT]

George A: May I also have the email address that was used?

me: [MY EMAIL 1]

George A: Well, I would need to send you a confirmation to that email
address. I would kindle need you to reply back to that email.

me: Please do

George A: Then, we will be able to delete that Skype Name for you.

me: thank you

George A: You are most welcomed, please expect me email within 10
minutes. Is there anything else I can help you with today?

me: Could you tell me if email accounts that are registered with Skype
are being verified by sending a message to them? If so, maybe there's
bug in your system?

George A: We send a welcome email to the registered email address
whenever a new account is set up using that email.

me: OK, that's what I received. And then you also send other emails
with offers to the same account. So, basically, anyone can create an
account for any email. Why don't you verify emails?

George A: Please understand that all of us here at Skype take our
customers' privacy and confidentiality very seriously

me: OK. Thank you.

George A: You are most welcomed. It's been a pleasure speaking with
you today. Thank you for contacting Skype Live Support, have a great
day. We value your feedback. Please be aware that we will ask you a
few questions after closing the chat window about your experience with
us today. Once you are ready please click on the "Exit" button.

me: I suggest adding a link to Welcome email that says "I didn't
create this account". Bye!

Realizing that there's nothing this support person can do about this, I sent email to their "security" people. I received no reply.

And now this failure to verify emails leads to the linked vulnerability. Nice.