Send Email to multiple Recipients with MailMessage?

Easy!

Just split the incoming address list on the ";" character, and add them to the mail message:

foreach (var address in addresses.Split(new [] {";"}, StringSplitOptions.RemoveEmptyEntries))
{
    mailMessage.To.Add(address);    
}

In this example, addresses contains "[email protected];[email protected]".

As suggested by Adam Miller in the comments, I'll add another solution.

The MailMessage(String from, String to) constructor accepts a comma separated list of addresses. So if you happen to have already a comma (',') separated list, the usage is as simple as:

MailMessage Msg = new MailMessage(fromMail, addresses);

In this particular case, we can replace the ';' for ',' and still make use of the constructor.

MailMessage Msg = new MailMessage(fromMail, addresses.replace(";", ","));

Whether you prefer this or the accepted answer it's up to you. Arguably the loop makes the intent clearer, but this is shorter and not obscure. But should you already have a comma separated list, I think this is the way to go.

According to the Documentation :

MailMessage.To property - Returns MailAddressCollection that contains the list of recipients of this email message

Here MailAddressCollection has a in built method called

   public void Add(string addresses)

   1. Summary:
          Add a list of email addresses to the collection.

   2. Parameters:
          addresses: 
                *The email addresses to add to the System.Net.Mail.MailAddressCollection. Multiple
                *email addresses must be separated with a comma character (",").     

Therefore requirement in case of multiple recipients : Pass a string that contains email addresses separated by comma

In your case :

simply replace all the ; with ,

Msg.To.Add(toEmail.replace(";", ","));

For reference :

1. https://docs.microsoft.com/en-us/dotnet/api/system.net.mail.mailmessage?view=netframework-4.8
2. https://www.geeksforgeeks.org/c-sharp-replace-method/