Evolution does not have (so far) "Flowing Text" mode where "the text is soft broken at the composer edge, but those soft breaks aren't translated to hard breaks when the mail is sent" ( Reference: https://mail.gnome.org/archives/evolution-list/2017-April/msg00079.html ) as in other editors or Email clients like Thunderbird.
You may have several alternative ways though.
Format the Email as Preformated for Plain Text Emails
The receiver side Email client and wrap the text as they prefer to.
The problem left is that the editing of the Emails for Preformated text in Evolution is painful. 2 possible solutions:
Edit the Email in an external editor (Hit Ctrl+Shift+E to open it) to edit the Email.
Edit the Email in "Normal" format (Ctrl+0), before sending the Email, select all and format them to be "Preformated" (Ctrl+7).
Edit and send the email in HTML
Use HTML Emails if you and your receivers are fine with that. HTML Emails are widely fine, especially in business world.
HTML text breaks are soft ones, and HTML markups force a hard line break. This is different from plain text where only hard breaks represented by the end-of-line character(s) are natively available.