Sending, receiving, and organizing email in Outlook.com
To better determine how the mailbox was modified, could you please confirm whether this is a personal Outlook.com/Hotmail/Live account or a Microsoft 365 work/school account? Also, when you say the email content was altered, do you mean that the message body itself has been changed, and are those same changes visible when you sign in directly at Outlook.com in a private/incognito browser and on other devices? If possible, please describe one example of what was changed, without sharing any personal or sensitive information. It would also be helpful to know whether the original recipient/sender has a copy showing different content from what you now see in your mailbox.
Regarding the possible remaining access and the rejected outgoing messages, please check
https://account.live.com/Activity
Let me know whether you see any unfamiliar successful sign-ins. Also confirm whether you have reviewed any connected apps/services or devices associated with the account. For the sending problem, please provide the exact rejection/bounce message and error code you receive, removing email addresses or other personal information, and let me know whether messages are rejected when sending to every recipient or only particular addresses/domains.