Hey Tor,

Rather than hooking into the EmailSendingNotification you'd probably be better off replacing the SendEmailTask pipeline task that is used as part of the send email pipeline.

This is the current pipeline task code

public class SendEmailTask : PipelineTaskWithTypedArgsBase<EmailSendPipelineArgs, EmailContext>
{
    private readonly IEmailSender _emailSender;
    private readonly ILogger<SendEmailTask> _logger;

    public SendEmailTask(IEmailSender emailSender,
        ILogger<SendEmailTask> logger)
    {
        _emailSender = emailSender;
        _logger = logger;
    }

    public override PipelineResult<EmailContext> Execute(EmailSendPipelineArgs args)
    {
        AsyncHelper.RunSync(() => _emailSender.SendAsync(args.Model.MailMessage));

        return Ok(args.Model);
    }
}

So you could tweak that to do what you need to do then check the docs here for working with pipelines https://vendr.net/docs/core/3.0.0/key-concepts/pipelines/ You'll want to call Replace<SendEmailTask, MySendEmailTask>() to swap the implementation.

Hope this helps