it seems that there is no plugin for thunderbird or outlook that allows you to check, even for a fee via api, if the email you are about to write exists. because nowadays sending an email to an existing address wastes time and money: 1) you think you sent an email and instead it has not arrived, creating a disservice to a customer who does not receive your email 2) the email sending system blocks you after 1 or a few sendings if you send N emails to non-existent addresses, because it is the typical problem created by spammers and from 1 to 30 non-existent emails sent in 1 hour and the mail server blocks you. 3) for example I had a similar problem with the school, and the limit of 30 is much lower in reality, because those emails bounce and try to resend for several days, I was blocked in a period in which I only had 10 orders per day and 20% of incorrect emails entered by customers.. another example: I have a customer who manages a hotel and in the past he had done improvised mass mailing and was blocked then today among the various emails, he received an email requesting a quote that was incorrect and with only one email he was blocked