Can someone being sued for insulting someone by email?

You can't be sued for insulting someone per se, but you can be sued for defaming someone.  Thus, if you called your landlord a jerk, that is not actionable.  Also, defamation does not occur unless you communicate the defamatory statement to a third party, or have reason to know that it will be disseminated to third parties, so private communications with your landlord are not defamatory.  However, if you accused him of stealing your money, and communicated that statement to any third party (such as by copying someone else on the email or posting your comment on facebook), the landlord might be able to sue you successfully for defamation.  Calling someone a "crook" might or might not be considered defamatory, depending on the circumstances and context.   Some jurisdictions also have some restrictions on some types of "hate speech", such as calling someone a racial epithet under certain circumstances.  Calling someone a crook would not seem to implicate such a statute (and anyway such a law may, if applied in certain circumstances, be invalid as violative of your first amendment right to free speech).