For a generic non personalized spam, IMHO it would be too expensive to generate and track millions of wallets. They could have placed a tracking pixel for much less (they didn’t, the email is just plain text)
If then it’s some targeted campaign, then yes, a dedicated BTC address makes sense as you said
please explain how to transfer bitcoin without mining a block, since the transactions are contained there.
You need to take the energy required to mine a block and validate it (a lot, could power a small town), then divide for the few transactions that could be included in just 1 mb.
They impose a size limit on the transactions that can be included, so even if tomorrow the transactions increase 10x, each block could contain the same limited number. Of course, if you only count the electricity used by your machine to send the transaction, it’s just a few milliwatts. The problem is all the garbage calculations that need to be done to actually validate it.