There aren't attachments limit really. However most servers limit the maximum size of the message.
You can use the SIZE extension if it is available:
using (Smtp client = new Smtp())
List<SmtpExtension> list = client.SupportedExtensions();
SmtpSizeExtension size = (SmtpSizeExtension)list
.Find(x => x == SmtpExtension.Size);
Assert.AreEqual(35882577, size.Value); // in bytes
The value returned is a maximum message size in octets. This number includes the message header, body, and the CR-LF sequences between lines.
You can use
IMail.EstimateTransportSize() to estimate the size of your message.