Who Sent Emails on a Subject (I am responsible for) to my manager who

I need help to find out who, on my behalf, sent email to my manager during my leave of absence. My manager is unavailable because he has now left company.
Without Login Credential of Manager who left Company, but with following criterion:
1. Mailbox owner is previously my manager (as per Outlook Organization)
2. Known Email Subject
3. Known Month of Email Sent
Is it possible to get email address of sender(s)?
Without login credentials of Manager who left company, is it possible to know the Senders of Emails (on certain Subject) to him/her?
BACKGROUND
Every month, I submit Report combining content of emails from other Departments by:
1. eMail as attachment to my manager
2. share attachment in above step 1 to a network location.
During my leave of absence, I received emails from other Departments but from my history of missing email, I am uncertain if there are subsequent emails to combine into a Submittal.
On the other hand, within my department, someone else did step 1 on my behalf but did not update me by any means (no email nor Team).
I am now faced with Step 2 without its attachment. I need help to contact the Senders of Report to my manager to get the attachment.

You will never find out unless you have physical access to the company servers.

This is about a task which is my scope of work; Subtask2 depends on output of Subtask1. Unknown did Subtask1 during my absence; I can't do Subtask2. IMHO shouldn't said email be automatically made available to person responsible?

To Find Email Sender with Criterion:
Subject of exact text
Sent Month
Can search be independent of Recipient Mailbox?

is Mailbox of Person who left company transferrable to someone else (who has authority)?

Knowing Sender Name allows me to contact him/her to let me have his/her output to finish partially done task during my leave of absence.

I believe my problem is common to contemporary workplace where turnover is higher in management than contract staff because of economic reasons. If IT can be offloaded and users can get information quicker, will that increase competitiveness of a organization?

I'll try to answer. LOL

Do you have access to the e-mails? Then look in the source headers.

Send the sender a canary token... One that doesn't obfuscate the IP address...