>Ever since I installed Outlook 365 and whenever I open it, I get a message Internet Security Warning (see msg 1 attached).
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>When I click on View button I get the Certificate window (see msg 2 attached)
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>I click on Install Certificate go through the Installation steps.
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>But then, on the next opening of Outlook, I get the same message of the Certificate.
>
>Is this certificate related to AV (Avast) or my email?

I am also getting that. I assume you have configured your account to use authentication for SMTP. Is that correct? This is what I have and had this a few times. Basically, even if you install, it will remove that procedure and re-ask the next times you will enter that software. Then, after a while, it will go away. This seems to act exactly like the SSL EV certificate on IE which reacts exactly the same.