Вот обсуждение на GitHub:
https://github.com/andyedinborough/aenetmail/issues/80
Почитайте, попробуйте пример оттуда, переделав под IMAP:
var fromAddress = new MailAddress(from);
var toAddress = new MailAddress(to);
var smtp = new SmtpClient
{
Host = "smtp.gmail.com",
Port = 587,
Timeout = -1,
EnableSsl = true,
DeliveryMethod = SmtpDeliveryMethod.Network,
UseDefaultCredentials = false,
Credentials = new NetworkCredential(fromAddress.Address, frompw)
};
using (var message = new MailMessage(fromAddress, toAddress)
{
Subject = subject,
Body = body
})
{
smtp.Send(message);
}