How to make a timer job run only once?

Question

I've written a code to run a timer job which runs at the start of a month informing users by email to visit the site.
Here's the code of the execute method:

public override void Execute(Guid targetInstanceId)
    {
        SPWebApplication webApp = SPWebApplication.Lookup(new Uri("http://server"));
        string smtpServerName = string.Empty;
        string from = string.Empty;
        //setting the website from where the timer job will run
        SPWeb web = webApp.Sites[0].RootWeb;
        //retrieving from address from the central admin site
        from = web.Site.WebApplication.OutboundMailSenderAddress;
        //retreiving smtpservername from the central admin site
        smtpServerName = web.Site.WebApplication.OutboundMailServiceInstance.Server.Address;
        //retreiving the groups in the website
        SPGroupCollection collGroups = web.SiteGroups;
        //logic to send mail to all users in all groups
        MailMessage mailMessage = new MailMessage();
        mailMessage.From = new MailAddress(from);
        string to = string.Empty;
        foreach (SPGroup group in collGroups)
        {
            foreach (SPUser user in group.Users)
            {
                //bool flg1 = user.Email == null;
                if (user.Email != null)
                {
                    //mailMessage.To.Add(user.Email);                        
                    to = user.Email + ",";
                }
            }
        }
        mailMessage.Subject = "Acknowledgement Mail";
        mailMessage.To.Add(to);
        mailMessage.Body = "Sup yo";
        mailMessage.IsBodyHtml = false;
        SmtpClient client = new SmtpClient(smtpServerName);
        client.UseDefaultCredentials = true;
        client.Port = 25;
        client.EnableSsl = false;
        client.DeliveryMethod = SmtpDeliveryMethod.Network;
        try
        {
            client.Send(mailMessage);
        }
        catch (SmtpException)
        {
            return;
        }
        catch (ArgumentNullException)
        {
            return;
        }
    }

Now just for testing purposes I ran this timer job every 5 minutes in an hour. Now, suppose there are 2 users in a group with email addresses a@abc and b@abc. I want the email to be sent with the "To" address being a@abc; b@abc; and I'm using smtp4dev for delivering the messages. What I saw was that the timer job had run twice in the 5 minutes and had sent 3 messages with the 1st being to a, then the 2nd being to a and b and the third being to a,b and again a. How do I make it run only once with only one message being sent regardless of the duration and only a;b in the "To" address?

EDIT: I forgot to restart my timer service after making changes in the code. It works now but the timer job runs multiple times within the scheduled interval instead of just once. Any suggestions for that? Sorry for messing this up!

Answer 1

My code ran when I ensured I had the following in place:
1) I forgot to restart the timer service after making changes to the code.
2) I used link1 and link2 to filter out duplicate users existing in more than one group.
3) Since I had more than 1 web application, the code for 1 application was being used for other web applications as well hence the repeated emails.
Sorry for asking this question and hope this helps someone someday.

Answer 2

This is because you are using mailMessage.To.Add(to); wrong. Each instance of MailAddress (MailMessage.To) can only hold a single mail address. What you need to do is to add each mail address separately (hence the .Add method).

What you will need to do is to uncomment your previously used code: //mailMessage.To.Add(user.Email);

See the following post withe exactly your problem, someone trying to use commas to separate mails.