In Outlook --> go to Tools --> Options --> Mail Format. There you will find HTML, Rich Text format or Plain Text format.
Generally the mail format is determined by the Sender (not the receiver). As the Receiver you can change the format, but it depends on your E-Mail software. The Sender has a mail format that he/she uses and probably rarely changes. Attachments are not impacted by this.
Attachments generally take the extension of the application that created the file -- *.doc == MS Word *.dat == Corel WordPerfect *.xls == MS Excel *.wks == Lotus123 etc.
Plain text format is just that... *.txt. It can be read with any software application including Word, notepad, WordPad, WordPerfect, etc.
Rich Text Format (RTF) is a file format that lets you exchange text files between different word processors in different operating systems.
Word Document Format (.Doc)is a proprietarty Microsoft format. It requires some version of Microsoft's Word to read.
Most e-mail readers will not recognize a Word.doc format. They will recognize text (txt), rich text (RTF) and hypertext markup language (HTML) formats.
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.doc vs. rich text format