I have 2 e-mail accounts - one on GMail and other one on a popular mail provider in my country (O2). GMail has a secondary e-mail set for security - that is the O2. I started getting e-mails from GMail -> O2 a while ago, their content was a regular spam. Now comes the thing that makes me puzzle:
O2 is my daily/primary account
GMail I use really seldom, I would say 3 times a year (job-seeking purpose) but I check it from time to time
Therefore there is (should be) no place on the internet where my GMail address is written, neither stored (only safe (?) companies). I also have never input this one into any Internet forms/subscriptions etc. But spoofing itself is understandable. What's not is how did someone connect these two accounts? GMail has literally 5 contacts, from which 2 are my other accounts, 3 other are companies. I haven't experienced spoofing in any other of my e-mail accounts.