I have set up the configuration as suggested in the documentation Inbound SMTP configuration properties | Alfresco Documentation and tried to do the example as suggested for inbound adding user to EMAIL Contributors and all that process. I am trying out this on my local system and I have got no errors to explain it further in the logs.
I see you have set the port to 25. Are you running Alfresco as root? If not, Alfresco cannot bind to port 25 because it is a privileged port. So if you are running as non-root, which you should be, this will have to be set to something higher, like 1025, and then you'll need to create a firewall port forwarding rule to forward SMTP traffic inbound from port 25 to port 1025.
Next, how does your email know to get to your Alfresco server? Have you created an MX record for your Alfresco server and domain so that it will be routed? If not, and some other server is the mail exchange for your email, has it been configured to route mail to the Alfresco server?
Have you checked to see if Alfresco is listening for SMTP traffic on the inbound SMTP port?
Have you opened up a thick email client, such as Mozilla Thunderbird and configured the outbound SMTP to point to your Alfresco server? This is a great way to test whether or not the Alfresco part of the equation is working.
Finally, make sure the sender's email address is on an authorized Alfresco user's person document, make sure that person is in the EMAIL_CONTRIBUTORS group, and make sure the sender has write access to whatever folder you are sending the email to.