Can anyone explain this…

OP

This is the tip of the iceberg, the real issue is your trust issues.

Unless you address it, something will trigger it again and you are back to square 1

email addresses are sold on, like telephone numbers.

Have you ever checked your spam folder? I was quite shocked when I saw how much spam I actually get that gets filtered out straight away so I never see it.

Well I am female and get mail email for penis enlargement and how to last all night!!! Spam email is the worst! Oh, and don't forget the millions that are owed me by some Prince in Africa.

I’d just ask him! But I know when I’m looking at certain things online my husband gets bits and pieces coming through all the time to his email

I think the issue here is that her husband was addressed by name, not the spam emails themselves.
I know that I occasionally get emails intended for the Bodach, as he only very rarely used the internet, so if something asked for an email address, he gave mine. These emails usually bear no resemblance to things where he might have given my address ( car insurance and golf supplies are two I can think of).
As her husband has his own email account, I agree it would be a bit daft to use hers if he was up to no good! I also suggest asking him.

Just ask him is a dangerous situation.

If the lack of OP's trust is because of her husband's past actions and she was gaslight, why trust his answer?

If OP's trust issues are not tied to his actions, he may get annoyed for implying he is cheating or wants to