Mojang account uses a gmail account that no longer exists, so any verification cannot be...

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today PseudoRandom

You're able to sign in properly, right?
The option to change your email address via Minecraft website is available?

In case you're not able to do it you have two options

1. Is try recovering the Gmail account associated with your Mojang account contacting Google support

2. Get in touch with Mojang support to see if you're able to change your email address providing enough ownership proof

Hope it helps, greetings from another Microsoft user!

Well, you may need to contact Mojang support to see if you have another option, or how you can proceed.

Due to you're stuck in their part of the migration progress

(Even if Microsoft owns Mojang, that doesn't mean both supports sites are the same, Mojang has their own internal processes)

I hope they can assist you further on this one

Unfortunately, the gmail account doesn't exist, so there is no way to prove my ownership of the account.

A couple years back I wanted to migrate my Mojang account to a Microsoft account. When I did I tied a gmail account an outlook account, but the outlook account already was used for a Mojang account. As a result, I can no longer access the account associated with the outlook address, instead it logs me into the account that was originally associated with the gmail. I looked on NameMC and found that the account still has a profile and I can see the right skin that I had on. This seems to suggest I could still log into the account, but when I try to login with the outlook on Minecraft,net and Mojang.com it directs me to my gmail Minecraft account or says that the account doesn't exist respectively. If there is any possibility that I could regain access to the original outlook Minecraft and Mojang accounts associated with the outlook account and unmerge the gmail and outlook I would like to know.

I need to know to

My son has a Mojang account connected with a mailbox that doesn't exist any more for a long time. A confirmation mail is sent to that mailbox. To migrate his Mojang account to his Microsoft account, this verification seems to be a necessary step.

How can I migrate the account without having access to that ancient mailbox.

There is a possibility to answer verification questions, like "What is your favorite food?" We don't know these answers any more.