Gmail has a way stronger spam filter than basically all other email account providers. Which is nice, for your actual personal accounts, but a huge pain in the ass when transferring a ton of accounts to someone else.
- Try to log in from a different location?
- Try to log in with a mediocre proxy?
- Do anything remotely suspicious?
Any of those things tend to result in getting locked out of a Gmail account until either doing phone verification or otherwise…