It sounds very natural to me.
Both 'sometimes' and 'sometime' can be used, but the meaning changes slightly.
They both ask you to message them, but the first is more a general comment/observation saying you need to message me from time to time.You need to keep in touch with me. The statement is implying that more than one future message is required.
The 2nd is more a request asking for a message. It doesn't have to be from time to time (like the first) or several messages, but you do need to message me at least once .