The user who reported this issue never got back to us, so we were unable to look into it.
But since you are already using the latest version, please give this a try:
1) Try to disable chunking, pipelining, and both before sending the message and let us know whether this has any effect:
smtp.EnabledExtensions &= ~SmtpExtensions.Pipelining;
smtp.EnabledExtensions &= ~SmtpExtensions.Chunking;
2) If this doesn't help, create a communication log using LogWriter property (see the code above) and either post it here or mail it to firstname.lastname@example.org for analysis.