Your box using a technique called greylisting to cut down on spam. Greylisting works by delaying mail from people you haven’t received mail from before for up to about 10 minutes. The vast majority of spam gets tricked by this. If you are waiting for an email from someone new, such as if you are registering on a new website and are waiting for an email confirmation, please give it up to 10-15 minutes to arrive.
-Every incoming email address also receives mail for +tag addresses. If your email address is you@yourdomain.com, you can also accept mail at you+anythinghere@yourdomain.com. Use this as a fast way to create aliases or to segment incoming mail for your own filtering rules.
Your box sets strict email sending policies for your domain names to make it harder for spam and other fraudulent mail to claim to be you. Only this machine is authorized to send email on behalf of your domain names. If you use any other service to send email as you, it will likely get spam filtered by recipients.