This may be coincidence, however just in case others have experienced the same;
I created aliases on my regular outlook account and used the sign-in preferences option to restrict logins to only a single alias which is never provided anywhere (E.g. shanelikesabargain_Youllneverguessmyalias@outlook.com) and my sign-in attempts dropped from hundreds a day to 0 for several months now.
After the above I recently created a new alias of regularemailtemp@outlook.com which i used to signup for comparethemarket exclusively. When i created a new alias I didn't realise it automatically adds it as accepting sign-in attempts and checked today and found regularemailtemp@outlook.com had a sign-in attempt. Using temp at the end of my regular email may be a coincidence that the email was guessed OR there's a leak from comparethemarket.
I did notice this thread from 2019 titled "Spam Emails After Using iSelect and/or Compare The Market" at https://www.ozbargain.com.au/node/489033 where OZB users mentioned getting spam after possibly using this service.
Generally speaking, companies like Comparethemarket make money through 2 ways;
1) commissions on the products they sell, and
2) selling your data
They are essentially data aggregators, with very signficant information about you as an individual (dob/address/estimated socio-economic status by the value of your vehicles, properties and contents). This is very valuable, and you agree for them to 'sell or share this information with its partners' - who then in turn, probably on-sell it again.
Eventually it'll end up in a questionable international company's hands (either used indiscriminately by them, or accidentally leaked)