we've been solving this issue over the e-mails (since Peter has a valid maintenance for his site) and I'd like to share our resolution for this issue:
In general, there are two possible ways of achieving the above mentioned aim:1)
You can populate the subscriber list within the newsletter module. This way it is not necessary to customize anything and you have a great control over them. Please see more details in our Developer's Guide: devnet.kentico.com/docs/devguide/subscriber_import_and_export.htm
If you don't want to import your subscribers to the Kentico CMS
database, you can amend the following code-behind: ~\CMSModules\Newsletters\Tools\Newsletters\Newsletter_Issue_Send.aspx.cs
within the btnSend_Click method:
As you can see, there is a chance to specify the e-mails manually in the second parameter of the SendIssue method. Basically, you can do that in any of those 3 if statements (in mean sending to all subscribers, scheduled sending or sending to specified ones).
You can define new SqlConnection as shown here: devnet.kentico.com/docs/devguide/displaying_data_from_external_database_or_web_service.htm
Please note, with such customization we can't guarantee that it will proceed all your 10000 smoothly. That's why I would still advise to import your uses to our Kentico CMS database. However, at the end the decision is up to you.