how to handle SINGLE_EMAIL_LIMIT_EXCEEDED, Failed to send email

here is my code ...please check it

trigger createactivity on Task (before insert,after insert,before update,after update) {
if(trigger.isbefore)
{
    if (Trigger.isinsert) {
 
     for(task t:trigger.new){
      if(t.status=='completed'){
          sendmailto();    
       }
      }
    }

}

if(trigger.isafter){

if(trigger.isinsert)
{
 for(task t:trigger.new){
      if(t.status=='completed'){
       sendmailto();    
       }
      }
}
 if (Trigger.isupdate) {
 
     for(task t:trigger.old){
      if(t.status=='completed'){
       sendmailto();    
       }
      }
    }

}
public void sendmailto()
{
     Contact c=[select id,lastname,Account.Name,email  from contact limit 1];
     List<id> contactids;
     
     string []to=c.email.split(':',0);
     system.debug('************************************'+to);
     if(c.email<>null)
     {
     Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
     
     email.setToAddresses(to);
      //email.setTargetObjectId(c.id);  
      email.setSubject('this will be the task subject');
      email.setplainTextbody('I spoke with '+c.lastname+ 'today from '+c.account.name +'about the latest proposal.');
      messaging.sendemailresult[]r=messaging.sendemail(new messaging.singleemailmessage[]{email});
      }
          
       else
        {
            Messaging.SingleEmailMessage mail1 = new Messaging.SingleEmailMessage();
            String[]  toAddresses = new String[] {'sivasalesforce.com@gmail.com'};
            mail1.setToAddresses(toAddresses);
            mail1.setplainTextbody('Invalid mail id'+c.email);
            //Messaging.sendEmail(new Messaging.SingleEmailMessage [] {mail1});
            messaging.sendemailresult[]r=messaging.sendemail(new messaging.singleemailmessage[]{mail1});

       }
     
}
}