MailBuilder.To is a collection - create and add a MailBox object for each email recipient:
MailBuilder builder = new MailBuilder();
builder.To.Add(new MailBox("first@example.com"));
builder.To.Add(new MailBox("second@example.com"));
builder.Text = "Hello";
....
IMail email = builder.Create();
Using fluent interface invoke To method for each recipient:
IMail email = Mail
.Text(@"Hello")
.To("first@example.com")
.To("second@example.com")
.From("from@example.com")
.Subject("Subject")
.Create();