Hello
I have this same problem, people are signing up OK but their email addresses are being deposited in the gmail account used for my wordpress web site instead of transferring into mailchimp. They’re not going into a spam folder in Gmail, just sitting in the in-list.
I am using ‘double opt in’
Unfortunately WordPress will not allow me to remove this gmail address without another, active email address being added. I’m having to transfer all email addresses and user names manually into mail chimp