Simplify.Mail using MailKit

[i4004]

Consider possibility of switching Simplify.Mail to MimeKit since it is not recommended to use by the official documentation: https://learn.microsoft.com/en-us/dotnet/api/system.net.mail.smtpclient?view=net-10.0&redirectedfrom=MSDN

We don't recommend that you use the SmtpClient class for new development because SmtpClient doesn't support many modern protocols. Use MailKit or other libraries instead. For more information, see SmtpClient shouldn't be used.