Validatable IEmailAddress

[asiffermann]

There is no email validation in Proffer.Email, except a very simple one for recipients in EmailSender (and only when mockup is configured? 🤷‍♂️):

proffer/src/Proffer.Email/Internal/EmailSender.cs

Lines 293 to 299 in b584d23

	string[] emailParts = recipient.Email.Split('@');
	if (emailParts.Length != 2)
	{
	throw new NotSupportedException("Bad recipient email.");
	}
	string domain = emailParts[1];

• Proffer.Email should provide a way to validate a IEmailAddress using a complete regex or System.Net.Mail.MailAddress.
• All email addresses arguments (recipients of all kinds, sender, reply-to...) should be validated with Guards.
• The options providing email addresses (default sender, mockup...) should be validated at startup.