Exchange 2013 installed on Hyper-V 2012 R2, with CU6, fresh installed, default config + 1 send connector. I cannot receive emails with attachments (connection died. broken pipe (#4.4.2).

Yes. Out of the box Exchange 2013 can receive mail. Like you mentioned you just need to create a Send Connector. By default I believe Exchange 2013 is configured for 35MB emails.

Have you only tested from Yahoo? Or have you tested from other sites as well? Is the error the same from multiple sources?

Can you run the Inbound SMTP Test on www.exrca.com and post the results here.

What do you have in front of your Exchange box on port 25? Any kind of anti-spam/anti-virus/message hygiene software, appliance or firewall? Can you remove any of those from mail flow for testing?

Is your firewall or any other appliance doing any kind of message hygiene or traffic shaping?

I have run the SMTP test and it looks fine (after I removed a secondary mx record that was pointing to an old IP).

In front of Exchange it is a firewall, but I presumed that it is ok because it was working with no problems with the previous mai server Exchange 2003. It is not a an anti-spam/anti-virus/message hygiene device, an I am using it just to forward SMTP port 25 internally, to the Exchange machine.

Here are the results:

Testing inbound SMTP mail flow for domain 'administrator@phoenixy.ro'.
 Inbound SMTP mail flow was verified successfully.
 
Additional Details
 
Elapsed Time: 10502 ms.

 
Test Steps
 
Attempting to retrieve DNS MX records for domain 'phoenixy.ro'.
 One or more MX records were successfully retrieved from DNS.
 
Additional Details
 
MX Records Host mail.phoenixy.ro, Preference 0

Elapsed Time: 349 ms.
 
Testing Mail Exchanger mail.phoenixy.ro.
 This Mail Exchanger was tested successfully.
 
Additional Details
 
Elapsed Time: 10144 ms.

 
Test Steps
 
Attempting to resolve the host name mail.phoenixy.ro in DNS.
 The host name resolved successfully.
 
Additional Details
 
IP addresses returned: 82.79.166.37
Elapsed Time: 1233 ms.
 
Testing TCP port 25 on host mail.phoenixy.ro to ensure it's listening and open.
 The port was opened successfully.
 
Additional Details
 
Banner received: 220 EX02.phoenix.local Microsoft ESMTP MAIL Service ready at Fri, 26 Dec 2014 20:12:19 +0200
Elapsed Time: 720 ms.
 
Analyzing SMTP Capabilities for server mail.phoenixy.ro:25
 SMTP Capabilities were analyzed successfuly.
 
Additional Details
 
SMTP Capabilities were analyzed successfuly.
250-EX02.phoenix.local Hello [134.170.52.123]
250-SIZE 37748736
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-STARTTLS
250-X-ANONYMOUSTLS
250-AUTH NTLM
250-X-EXPS GSSAPI NTLM
250-8BITMIME
250-BINARYMIME
250-CHUNKING
250 XRDST
 
Elapsed Time: 708 ms.
 
Attempting to send a test email message to administrator@phoenixy.ro using MX mail.phoenixy.ro.
 The test email message was delivered successfully.
 
Additional Details
 
Elapsed Time: 2117 ms.
 
Testing the MX mail.phoenixy.ro for open relay by trying to relay to user Admin@TestExchangeConnectivity.com.
 The Open Relay test passed. This MX isn't an open relay.
 
Additional Details
 
The open relay test message delivery failed, which is a good thing.
The exception detail:
 Exception details:
Message: Mailbox unavailable. The server response was: 5.7.1 Unable to relay
Type: System.Net.Mail.SmtpFailedRecipientException
Stack trace:
 at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)
 at System.Net.Mail.SmtpClient.Send(MailMessage message)
 at Microsoft.Exchange.Tools.ExRca.Tests.SmtpOpenRelayTest.PerformTestReally()

Elapsed Time: 5353 ms.

Hmm. I would check the SMTP Send and Receive Logs.

You should find it in the Exchange Install Directory.

For example, C:\PROGRAM FILES\MICROSOFT\EXCHANGE SERVER\V15\TRANSPORT ROLES\LOGS\PROTOCOLLOG\

You may need to turn on protocol logging first. More info on that here.
http://technet.microsoft.com/en-us/library/aa997624(v=exchg.150).aspx
http://technet.microsoft.com/en-us/library/bb124531(v=exchg.150).aspx

Then send some test messages.

Exchange 2013 in a Hyper-V 2012 R2, fresh install, is working by default. For a small organisation with under 50 users, it wil work ok with the minimum recommended (2 proc / 8 GB RAM). The single thing need to be done is to creat a send connector for sending emails to internet. That is for a fast start in a production enviroenment. The next steps: install a certificate for https, misc. configurations, etc. could be done in after-work hours ...