Why did the mailing list often end up in spam?

Question

Email Marketing

Good day! This is a question that has been bugging me for about a month now. I'm doing a mailing list for my client (she works for a direct selling company).
Postal addresses are left by the users themselves in a paper questionnaire. Over the past month, my mailing list has been frequently spammed, and I receive messages about it:
Unfortunately, your email could not be delivered to one or more recipients:
[email protected] account is disabled
The error that the other server returned was:
550 5.7.1 No such user!
And the like. Users definitely exist and before they received letters. It's the same with rambler.ru and Yandex, and even if you send the newsletter not from gmail.com, as I usually do, but from mail.ru.
From time to time, messages come from mail.ru that you need to follow the link and confirm that I do not send spam. Everything worked fine before. What's happening?

Answer 1

It seems to me that you are tired of those people who are subscribed to the mailing list, and they just mark the letter as spam =)