SMF has a built-in email sender, which can send emails to all members and specific groups. There is also a built-in newsletter, so people will know even if they are not logged in to the forum. I'm also a member of some SMF community, and they use it to send promotions.
The only issue is that the email could fail because of the hosting's hourly and daily email sending limit.
I doubt that such limitations pose a problem, given that this forum uses a highly customized hosting solution, or most likely, a dedicated server.
That said, raw email limits are only half the battle... the real nightmare with SMFs default mailer is deliverability. Gmail and Outlook will probably flagg the whole batch within minutes, and vast majority of the users will never even see the messages. Not even in spam.
TBH, any admin who has been around the block sets up an external SMTP relay like Mailgun or Amazon SES to handle things like this.
Costs practically nothing. Saves you from having your server IP dumped onto every spam blacklist on the planet, which takes forever to clean up...