EmailAccount.cs

   public class EmailAccount
    {
        public string Username { get; set; } = default!;

        public string Password { get; set; } = default!;

        public string Email { get; set; } = default!;

        public string DisplayName { get; set; } = default!;
    }

EmailAttachment.cs

  public class EmailAttachment
    {
        public string FileName { get; set; } = default!;

        public Stream Content { get; set; } = default!;

        public string ContentType { get; set; } = default!;
    }

EmailMessage.cs

public class EmailMessage
    {
        public string Subject { get; set; } = default!;

        public string Body { get; set; } = default!;

        public IEnumerable<EmailAttachment> Attachments { get; set; } = Array.Empty<EmailAttachment>();

        public IEnumerable<string> Recipients { get; set; } = Array.Empty<string>();
    }

IEmailSender.cs

  public interface IEmailSender
    {
        Task SendEmailAsync(EmailAccount account, EmailMessage message);
    }

MailKitEmailSender.cs

   public class MailKitEmailSender : IEmailSender
    {
        private readonly IOptions<MailKitEmailOptions> _emailOptions;

        public MailKitEmailSender(IOptions<MailKitEmailOptions> emailOptions)
        {
            _emailOptions = emailOptions ?? throw new ArgumentNullException(nameof(emailOptions));
        }

        public async Task SendEmailAsync(EmailAccount account, EmailMessage message)
        {
            var minme = new MimeMessage();
            minme.From.Add(new MailboxAddress(account.DisplayName, account.Email));

            foreach (var recipient in message.Recipients)
            {
                minme.To.Add(new MailboxAddress(string.Empty, recipient));
            }

            minme.Subject = message.Subject;

            var builder = new BodyBuilder();
            builder.HtmlBody = message.Body;

            foreach (var attachmentInfo in message.Attachments)
            {
                builder.Attachments.Add(attachmentInfo.FileName, attachmentInfo.Content, ContentType.Parse(attachmentInfo.ContentType));
            }

            minme.Body = builder.ToMessageBody();

            using var client = new SmtpClient();

            if (_emailOptions.Value.EnableSsl)
            {
                await client.ConnectAsync(_emailOptions.Value.SmtpHost, _emailOptions.Value.SmtpPort, SecureSocketOptions.SslOnConnect);
            }
            else
            {
                await client.ConnectAsync(_emailOptions.Value.SmtpHost, _emailOptions.Value.SmtpPort, SecureSocketOptions.StartTls);
            }

            await client.AuthenticateAsync(account.Username, account.Password);

            await client.SendAsync(minme);
            await client.DisconnectAsync(true);
        }
    }