I'd double check the admin directory as well, and also maybe check the php version on the new host. If all of that checks out, then we might end up needing a support ticket since I'd need to do some direct debugging.
What's happening is when sending via BCC, it builds an array of email addresses.
"email1@domain.com","email2@domain.com","email3@domain.com"
We then grab each one, array[0] would be email1, array[1] would be email2, and so on.
On your server, when we grab array[0], it's just returning the e instead of the full email. That will happen if it thinks the whole thing is a string, so [0] would specify the first character of that string.
