os_smtp_* errors, e.g., os_smtp_open, os_smtp_read, or os_smtp_write errors, are not PMDF errors per se: they correspond to PMDF reporting back about a problem encountered at the network layer. For instance, an os_smtp_open error means that the network connection to the remote side could not be opened, which can be due to addressing errors or channel configuration errors (PMDF configured to attempt to connect to the "wrong" system), but is more commonly due to DNS problems or network connectivity problems (particularly if this is a channel or address that was previously working). os_smtp_read or os_smtp_write errors are usually an indication that the connection was aborted (either by the other side or due to network problems).
Note that network and DNS problems are often transient in nature. It is normal to occasionally see such problems. Indeed, for connections to troublesome systems, it can even be common. So the occasional such error is usually nothing to be concerned about. However, if you are consistently seeing such errors on most messages on a channel, or seeing such errors on most messages to or from a particular remote system, then the errors can be an indication of an underlying network problem.
If you need more information about an os_smtp_* error, enable debugging on the channel in question and get a debug channel log file showing details of the attempted SMTP dialogue; see Section 188.8.131.52. In particular, the timing of exactly when a network problem occurred during the SMTP dialogue tends to be suggestive as to what sort of network or remote side issue might be involved. In some cases, you can also want to do network level debugging (e.g., TCP/IP packet tracing) to see what was sent or received over the wire.