XML Bursting emails not delivered to few external email Ids

Hello,

We have a email payslip program. Employees can prefer to have the payslips emailed to their personal Email Ids ( Hotmail, Gmail etc). The program is an XML Publisher program with Bursting.

From few months few of the employees have not received their payslips. The XML Bursting program log has no SMTP error messages. The emails do get delivered if the email ids are changed to an internal email id ( company id). There are few other employees who use personal email IDs but have no issue receiving this emails.

Please advise. we are able to replicate the issue on a Non Prod environment.