When you send contacts to a list to MailChimp, you will get an email from Method giving you a summary of the export:
This email will inform you when a contact is not sent.
Method will not a send a contact to MailChimp if:
Currently, at this time, there is no way to override this filter.
There are a few reasons why MailChimp will not accept a contact, and this mirrors the same import errors you would get if you were importing contacts directly in MailChimp.
If you are a paid subscriber to MailChimp, you will receive an error when you send contacts to MailChimp if you surpass your limit of subscribers.
There are two things you can do in this situation: