OP literally described a timescale for (not spam) to (spam) and you described a few possible options (basically a knee jerk reaction, with no evidence).
The medical world uses the terms indications and contraindications. I think we in IT could do with a similar process/procedure.
I'm sorry that I'm disputing their totally scientific "timescale". Btw did you mean timeline?
It can take a while for spammers to get your address on their list. Generally the amount of spam over time only goes up. Just because there's a correlation doesn't mean there is a causation. Anyone with an email address, including on a custom work domain, has experienced this and Claude wasn't responsible for all email spam lol
From somebody who helped fixing bugs by contacting customers directly: the first thing you need to throw out from their story is correlation. Most of the time, it's just a coincidence.
For example, my only used by Claude email address doesn't get any spam at all.