articles by different authors

I need to publish several articles from different authors.
Of course, I can send from my own address, but I need the articles to be placed in the correct categories and attributed to the correct author. In my tries: the emails are received and processed, but the article is still attributed to the Default Poster; the categories are not detected correctly; the text doesn’t retain its formatting; the attachment is correctly used as a featured image.
Can you suggest me how to proceed?

The page I need help with: [log in to see the link]