As part of our process we send emails to users and/or groups. We use OOTB Send E

What does the mail look like?

The mail contains from address, subject and content properly. But the To/CC fields are empty (though the mail gets delivered to the recipients properly)

Is this cloud or on premise. I'm assuming it's on premise.. is so, is the email connecting to your internal email gateway? Are there any rules or policies in there ?

Can you try configuring the email to use anything like Gmail to see if the issue continues to persist?

Hi Sathya, it is premise only. And to add more info, if the To/CC fields are populated with user datatype, then To/CC recipients are visible in the delivered mails. But if the To/CC fields are populated with Groups, then To/CC recipients are invisible in the delivered mails.
Eg. To: senthil (user); sathya (user). In my mail I see both senthil and sathya populated in the recipients.
Eg2. To: officers (Group). In this case, I get the mail delivered to me as I am part of officers group. But the recipient area in the mail is empty.

Is that an issue?

Groups exist in Appian, but do not have email addresses associated with them. When you send an email to a group it sends email to each of the Appian users, which have email addresses associated with them.

Could you provide the MIME content of the message?

Thank you Michael!