Since how many emails VSE considers as mass mailing and block TCP/25 port when this feature is enabled?
I don't think VSE imposes limits by 'how many emails' but rather by process names. VSE by default blocks port 25 use except for those programs explicitly listed. Everything else is considered suspect as a 'mass mailer' and blocked.
Normally, all processes are blocked from using port 25, with Exceptions, such as Outlook.exe. So, in this case, Outlook is allowed to send email via port 25, simply because it has been Excluded from the blocking that is otherwise in place. (There are many applications listed in the Exclusions section; I simply used Outlook as an example.)
Another process name, say, Seamonkey.exe, which is not explicitly Excluded, will get blocked.
The point of this is to stop all but specifically allowed (Excluded) processes from any access to mailing via port 25. This prevents unknown or unauthorized applications from suddenly getting control and spamming the world.
I have seen ISPs implement limits on the number of people listed in the CC or Address fields when mailing. The limit may simply be, how many messages sent within a certain time frame, or a combination of messages with CC together. This is the ISP that is imposing limits to keep people from sending out too much email - even intentionally - to too many people at once. This protects the ISP from getting listed in an RBL.
For those doing real and legitimate marketing and/or sending of opt-in newsletters, there are services designed specifically for this purpose.
I hope this answered your question; post back if you need more info.
Thanks a lot, Ron!
Your Welcome, glad to help.