I have around 1 thousand users and I tried sending a mass email to all my users.
After taking a good time, while sending the mass email my page stopped loading & stopped working. It did not send the mail to all the users successfully.
The reason is shared hosting servers do not allow mass mailing, so I would suggest would be great to send mass emails to the users batch by batch. There should be an option to choose sending mass email to users a batch of 100 users first and then 100 users next. Something like that, it will help everyone specially shared hosting users and they will be able to send mass emails to all the site users.