How do I format a String in an email so Outlook will print the line breaks?

You can force a line break in outlook when attaching one (or two?) tab characters (\t) just before the line break (CRLF).

Example:

This is my heading in the mail\t\n
Just here Outlook is forced to begin a new line.

It seems to work on Outlook 2010. Please test if this works on other versions.

See also Outlook autocleaning my line breaks and screwing up my email format

You need to use \r\n as a solution.

I've just been fighting with this today. Let's call the behavior of removing the extra line breaks "continuation." A little experimenting finds the following behavior:

• Every message starts with continuation off.
• Lines less than 40 characters long do not trigger continuation, but if continuation is on, they will have their line breaks removed.
• Lines 40 characters or longer turn continuation on. It remains on until an event occurs to turn it off.
• Lines that end with a period, question mark, exclamation point or colon turn continuation off. (Outlook assumes it's the end of a sentence?)
• Lines that turn continuation off will start with a line break, but will turn continuation back on if they are longer than 40 characters.
• Lines that start or end with a tab turn continuation off.
• Lines that start with 2 or more spaces turn continuation off.
• Lines that end with 3 or more spaces turn continuation off.

Please note that I tried all of this with Outlook 2007. YMMV.
So if possible, end all bullet items with a sentence-terminating punctuation mark, a tab, or even three spaces.