3 Lahk is 300,000? I don't think the Newsletter widget has been tested with that many email addresses.
The emails are sent out a background thread -- so they won't hold up anyone using the website.
One thing is, will your mail server allow you to send out that many.
Another consideration is app pool recycles. By default application pools recycle every 29 hours. If the process to send the emails starts near the scheduled app pool recycle time, then the email sending will be interrupted and the unsent emails
will not be sent out. Or if the website restarts or the server restarts for any reason, the process will be interrupted and any unsent emails will not be sent out.
The newsletter widget isn't advanced enough that it has a queue and continues on sending emails that weren't sent earlier. If you have that many email addresses, or any large number, it'll be safer to use a more robust email/newsletter management system.
Whenever you create a new post, you could use that newsletter management system to notify everyone a new post is available.