I found this link very useful for Windows phone 8.1
Make a class like MailManager in data model folder and add this line of code at the top of your class using Limilabs.Mail;
also add this Mail.dll
library to your project. Read more
StorageFile imageFile = ...;
StorageFile attachmentFile = ...;
MailBuilder builder = new MailBuilder();
builder.Html = @"Html with an image: <img src="" cid:lena@@example.com""="">";
MimeData image = await builder.AddVisual(imageFile);
image.ContentId = "lena@example.com";
await builder.AddAttachment(attachmentFile);
builder.To.Add(new MailBox("to@example.com"));
builder.From.Add(new MailBox("from@example.com"));
builder.Subject = "Subject";
IMail email = builder.Create();
using(Smtp smtp = new Smtp())
{
await smtp.Connect("smtp.server.com"); // or ConnectSSL for SSL
await smtp.UseBestLoginAsync("user", "password");
await smtp.SendMessageAsync(email);
await smtp.CloseAsync();
}