somehow one line of code is skipped

Stop using @setfield.  It's only useful within loops (if you use R6) or within an @do structure (R5).  @setfield requires that the field already exist, and is the cause of a lot of problems, if you don't use it correctly.
Also watch out using the @author function.  It doesn't always returns the original author.  If you want it to be the name of the current user, use @username.

Try again :

FIELD regHis :=  "On " + @Text(@Now) + ", " + @name(]cn];@username) + " wrote " + rejectReason;
FIELD regStatus :=  "On " + @Text(@Now) +",  " + @name(]cn];@username) + " rejected.";
@MailSend(user1;user2;"";"Rejected" ;"";"Rejected.";[IncludeDoclink]);
FIELD MAIL_SEND :=  "Y";
@Command([FileSave])

cheers,

Tom

About @setfield : if you use R6, it should also work when the field doesn't exist yet, but it remains good practice to make sure it exists :

This is the correct way of using @setfield :

FIELD MyField := MyField;
...
@setfield("MyField";"some value");
...

Tom,
thanks for that.
However if I do not give any value to the rejectReason field
then the mail is still sent out.
What i am trying to achieve here is that i want to make
sure the rejectReason field is being filled with content
else the mail is not allowed to be sent.

Thanks!

Try this

@SetField("regHis"; "On " + @Text(@Now) + ", " + @Author + " wrote " + rejectReason);

@SetField("regStatus"; "On " + @Text(@Now) +",  " + @Author + " rejected.");

@MailSend(user1;user2;"";"Rejected" ;"";"Rejected.";[IncludeDoclink]);

@SetField("MAIL_SEND"; "Y");

@PostedCommand([FileSave]);

~Hemanth