return column values based on conditions in R

Question

I have 2 columns from my data table where, i need to return the mail IDs where the mark is less than 50.

When tried with the below code, getting error that "Multiple columns returned for vector output".

check<-function(R)
{
 s<-list(M)
 if(R > 50) 
 {
 s<-list(M)
 }
 return(s)
}

out<-sapply(R,check);