As you can see from looking at your question, text formatting is likely to screw up any attempt to do this in a plain text email. (Most email clients don't use a fixed pitch font and, in plain text emails, you can't specify markup to change that.)
So you have two options:
- Use HTML mail, and construct a genuine table (with <table><tr><td> etc)
- Change your format so that it works well in a plain text message, perhaps "A:(tab)5 occurrences(newline)B:(tab)6 occurrences".
Use a StringBuilder to construct the text, either way.