What is important in a "message center"?

I code a webapp for online classifieds in India. The app is also supposed to have a "message center" where you can send and receive messages. I have implemented sending and receiving messages, profiles, user model and listing messages.

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Now I wonder if I "thought of everything", I have not implemented the following

  • Sending to a username is not implemented. The identified is the email of the user. Could this be a problem, if users change their emails I probably should use the user ID instead.

  • Sending to several users the same messages is not implemented. I think that it is not important for a minimum viable version.

  • Attaching images is not supported.

  • HTML messages are not supported.

Did I miss anything, did I include something unneccessary or otherwise doing it wrong? What should I implement next?

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