EXM - some recipients missing email on form submission

In Sitecore 9.1.0 we have an automated email campaign where 4 recipients are supposed to receive an email on form submission but on submission-only 2 of them are receiving the email and the rest 2 are skipped according to the EXM logs with below message -

The sender’s email is invalid. 

This might prevent successful dispatch But these senders are getting emails on QA for the same campaign and on prod with the different campaigns and all the SMTP and configs same for QA and prod.