There is no such kind of restriction.
Can you please create communication logs for the two cases and send the log files to email@example.com for analysis (or post it here).
It should show the differences between the two cases.
The log file can be created like this (in MailForm.vb constructor):
_smtp = New Smtp
_smtp.LogWriter = New FileLogWriter("c:\data\smtp.log.txt", LogLevel.Debug)