It is possible to use email addresses ending in certain TLDs meant for testing. For example: user@mydomain.invalid, user@mydomain.example, etc.
I am not sure how MailChimp handles those email address. If is is already handled then please guide to relevant documentations. If not handled already, then please bring code change to prevent those email addresses being sent to MailChimp.
Active
2.0
Lists Module
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The code added in the patch comes after a call to mailchimp_lists_load_email(), which is already trying to validate the email address. Would it make more sense to put your improved validation in mailchimp_lists_load_email() instead of mailchimp_lists_process_subscribe_form_choices()?