I have tested the warm-up feature in Mailead on an email account that had already been warmed up for months with Instantly (and is still actively warming up there). This account hasn’t been used for any other purposes. However, after activating the warm-up in Mailead, the health score dropped drastically—from 100% in Instantly to 69% in Mailead within just one week.

Specifically, 5 emails landed in spam out of only 16 emails sent. This is concerning for a previously healthy account where emails almost never landed in spam.

I don't know what signals in Mailead's warm-up emails are triggering spam filters, but I believe identifying and fixing this issue should be a top priority.