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Massive X data leak affects over 200 million users.
(www.techzine.eu)
This is a most excellent place for technology news and articles.
How do you reply to those emails in case of needing to contact with said company.
I'd assume they would deny service if the user (even on the same custom domain) is not equal to the account holder.
When you get an email from Company A that sends to your alias email, the email goes to your inbox. When you reply to that email, your alias provider forwards it to Company A where the sender is your alias address.
In short, you simply reply and your alias service takes care of it for you so that the recipient only sees your alias email and not your true email.
I don't use an "alias provider".
I just don't use aliases for companies I need to send emails to. There are very few.