The proper Dutch way of greeting someone at the end of a message would be "groetjes" (informal) or "groeten" (more formal) or "gegroet" (formal and a tad old-fashioned). Just "groet" is wrong and looks odd. "Groet" is only used as a (singular) noun, as in: "Een groet van hem" ("A greeting from him").
Anyway, do you have any idea of how standoffish you come across in your messages? Maybe you're depressed, lonely or have autism? If so, just let us know so that we understand you better and know better how to interact with you. :)