R12.1.3 & Office 365 -Standard Purchase Oder Approval not working outside company network

Good Morning ,

We are using R12.1.3 with Linux . So far workflow was working fine. Before our exchange server was in in our office building only. This month we changed to OFFICE 365 ( exchange on cloud)

After Changing to Office 365- we facing below 2 issue - For example Standard Purchase Order Approval.

Question 1:

After the buyer submit PO for approval, approver receiving notification properly -

If the user is not in our company network , some time user are telling that they are not receiving the Email Notification.

But in Table -WF_Notifications -Mail_Status shows value =SENT

Even though shows SENT- still few users not receiving Email Notifications.