I'd believe it. For what it's worth, something like 80% - 90% of all email is spam. It's usually better to leave it there and cordon it off so that real users don't notice it. Spammers don't get feedback that their spam isn't being shown to real users.
Also 350,000 accounts is hardly "massive" as the clickbait title suggests. No way that can account for more than a small percentage of traffic.
It's possible that the majority of spam is not meant to be seen by humans, but is instead designed to train spam filters and keep them busy, so some payload still penetrates the shield.
Also 350,000 accounts is hardly "massive" as the clickbait title suggests. No way that can account for more than a small percentage of traffic.