Some people do not get a confirmation email

I have had 3 people that did not receive a confirmation email 2 have gmail address and one had address. Some with gmail did receive confirmation emails. I had them check spam folder and the emails did not show up there (at least 1 with gmail did check).

Has anyone else had this issue? what can be done to fix it or is it a setting with their email address itself?

I did manually confirm them all so the next question is when I send a campaign will they also have an issue getting those emails?

one additional note. I was helping someone with theirs and I tried to update their subscription instead of adding another newsletter subscription and they did get an email about that so funny they did not get one for confirmation.

any help or information would be appreciated.

thanks for your time in advance.