To quickly eliminate unwanted linebreaks from text, select (highlight) the passage, right-click it, then choose "Message Plugins > Unwrap text". Note: if you are doing this to quoted text in a reply, the quoting will be lost (but see below).
By right-clicking your toolbar and choosing "Customize", you can add a button for this feature, it is on the "Plugins" tab of this dialog.
Another useful message plugin allows you to quickly change the case of any highlighted text passage. You can switch to all lower or all upper case, initial caps per word or per sentence, and even toggle the case of all selected characters.
When replying, the indentation level (quoting level) of previous message text
is crucial to let others understand who is replying to what. Selectively quoting
only the interesting passages is good email etiquette - delete everything irrelevant
before writing your reply. Sometimes the quoting gets messed up, but there is
a quick way to fix that: highlight the text in question, then type Ctrl->
(control+right angled bracket, on US keyboards that's Ctrl-Shift-period) to
indent (quote) the text, Ctrl-. (control+period) to unindent it
(remove quoting).
Modern mail clients offer many features for formatting your outgoing mail, internally done via HTML markup. Eudora calls this "styled text". You can highlight and colorize text, even embed images. However, many recipients prefer reading their mail as plain text or have mail clients that cannot handle HTML mail at all. Email etiquette still recommends to send plain text unless you are sure the other side can handle HTML mail. Never send styled text to a mailing list (LISTSERV)!
By default, Eudora sends messages as styled text. You can turn this off via "Tools > Options > Styled Text". Select "ask me each time" if you like sending HTML mail occasionally. If you don't even show the formatting toolbar, all your mail will be plain text. The option to send both plain text and styled text is generally frowned upon.
When receiving HTML mail, sometimes you may want to look at the source code
of the page, especially when Eudora has problems displaying something right.
You can right-click in the message body and select "View Source".
You can also save out the current message as a separate .txt file
via "File > Save As", then open it in the program of your
choice.