I'm not sure about threshold but you absolutely will get flagged as spam or rejected if you aren't following bulk-sender guidelines for mass emails. Most major email service providers put out a list of these guidelines and they should be adhered to if you want your mail to be delivered.
Google's can be found here for example:
The guidelines in this article will help you successfully send and deliver email to people who have personal Gmail accounts. This article was previously called Bulk sender guidelines. Important: T
support.google.com
Outlook's:
You also should be aware of the CAN-SPAM Act and Unsubscription requests.
Do you use email in your business? The CAN-SPAM Act, a law that sets the rules for commercial email, establishes requirements for commercial messages, gives recipients the right to have you stop emailing them, and spells out tough penalties for violations.
www.ftc.gov
If users are requesting to be removed/unsubscribed or if they do not have a method to do this you are in violation of this and could potentially be subject to a fine.
Bad email addresses could be spam traps but regardless you shouldn't be wasting time sending email to addresses that aren't receiving your mail because the address is no longer valid. All of the above are why many folks use an email campaign or mass mailing service to do business like this.