An incomprehensible increase in mailing spam?

Question

Email Marketing

There is a corporate mailing system - written in Yii (we use PHP Mailer)

Literally since the new year, 5-6 thousand kickbacks began to come for 10,000 sent letters - mainly from spam filters.

We did not change the mailing list - everything at the same time - in the same style -
checked the ip-address of the mail domain with the same frequency - there are no blacklisted spam
letters sent to mail-tester.com - gives a rating of 10 out of 10
Hosting- the provider makes a helpless gesture...

Where to dig? What else can be checked?