how have hackers got my Plusnet logon email address?

[#1]

I login to Plusnet using a special email address I created for Plusnet ONLY and is not used for any other purpose.
This email address has
    never been written down,
    has never sent any email,
    it has not been pwned
So it is known only to my memory and Plusnet.

I have just got one of those phishing emails that says they have webcam footage of me and I need to transfer bitcoins to them or they will send this video to my contacts.
I am not concerned about this (I don't even have a webcam for them to record) but I am concerned that it has been sent to, and refers to, my Plusnet logon email

I can not think of anyway hackers could get this email unless plusnet has been hacked.

[#2]

There is a probable chance that they haven't 'got' your email address. Much of the spam is sent to emails generated by the spammer's software. If they guess correctly, the email gets delivered, and perhaps gets placed on a email list. If they get it wrong it simply bounces. With so many botnets out there, it isn't exactly the most difficult of operations for even a beginner.

All that is needed is a target, such as yahoo, or gmail etc.

There are other ways too, dodgy browser addons or unwanted programs slurping your data or even malware or a virus. If you used your phone or tablet to manage your email, there maybe something tucked away sending this info. I would assume that you are keeping things up dated and run malware and virus scans on your devices, so I think this would probably be an unlikely source.

I really wouldn't worry too much, but you're right to bring it up so it is flagged.. just in case

[#3]

Hi there, 

As GJenkins mentioned unfortunately a lot of spam comes purely from guesswork, either entirely or finding the domain and guessing at mailbox names. 

I would definiltely make sure all your security software is up to date and run full scans.