Try to find related error messages in maillog. You can find maillog location here -
http://kb.parallels.com/en/111289
Thanks
SMTP Activity Log :
06/09/12 03:31:28 SMTP-OU CA033E2B76EB465AA067E3E7D28A20DE.MAI 604 98.139.54.60 CONN 421 4.7.1 [TS03] All messages from [myip] will be permanently deferred; Retrying will NOT succeed. See
http://postmaster.yahoo.com/errors/421-ts03.html 0 162
06/09/12 03:31:28 SMTP-OU CA033E2B76EB465AA067E3E7D28A20DE.MAI 604 98.139.54.60 QUIT QUIT - 6 0
06/09/12 03:31:28 SMTP-OU CA033E2B76EB465AA067E3E7D28A20DE.MAI 604 72.30.235.196 CONN 421 4.7.1 [TS03] All messages from [myip] will be permanently deferred; Retrying will NOT succeed. See
http://postmaster.yahoo.com/errors/421-ts03.html 0 162
06/09/12 03:31:28 SMTP-OU CA033E2B76EB465AA067E3E7D28A20DE.MAI 604 72.30.235.196 QUIT QUIT - 6 0
06/09/12 03:31:28 SMTP-OU CA033E2B76EB465AA067E3E7D28A20DE.MAI 604 66.94.237.64 CONN 421 4.7.1 [TS03] All messages from [myip] will be permanently deferred; Retrying will NOT succeed. See
http://postmaster.yahoo.com/errors/421-ts03.html 0 162
SMTP Debug Log :
06/09/12 03:31:28 ME-I0018: [EFFD15BBB95E4C4284EE1D5772C751B1.MAI] Outbound message from ([SMTP:test@mydomain.com]) requeued as [CA033E2B76EB465AA067E3E7D28A20DE.MAI] to the target domain [ymail.com]
06/09/12 03:31:28 ME-I0123: Domain [ymail.com] has MX list [mta7.am0.yahoodns.net,mta5.am0.yahoodns.net,mta6.am0.yahoodns.net]
06/09/12 03:31:28 ME-I0026: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Sending message
06/09/12 03:31:28 ME-IXXXX: [SYSTEM] DNS resolved to the following record: IP Address=98.139.54.60, Family=2, Type=1, Protocol= 6
06/09/12 03:31:28 ME-I0009: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Remote server has closed connection after 0 milliseconds. Server Response: (-)
06/09/12 03:31:28 ME-I1350: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Attempting to connect to MX 2 of 3 (mta5.am0.yahoodns.net).
06/09/12 03:31:28 ME-IXXXX: [SYSTEM] DNS resolved to the following record: IP Address=72.30.235.196, Family=2, Type=1, Protocol= 6
06/09/12 03:31:28 ME-I0009: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Remote server has closed connection after 0 milliseconds. Server Response: (-)
06/09/12 03:31:28 ME-I1350: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Attempting to connect to MX 3 of 3 (mta6.am0.yahoodns.net).
06/09/12 03:31:28 ME-IXXXX: [SYSTEM] DNS resolved to the following record: IP Address=66.94.237.64, Family=2, Type=1, Protocol= 6
06/09/12 03:31:28 ME-I0009: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Remote server has closed connection after 0 milliseconds. Server Response: (-)
06/09/12 03:31:28 ME-E0059: [CA033E2B76EB465AA067E3E7D28A20DE.MAI] Message Delivery Failure. Attempt (3): Domain (ymail.com) returned temporary error for email. Message has been requeued.