I want to send 1 message to all email ids in webmail is it possible ?
is it possible? Probably. Depends on what webmail system you’re using, and what you’re attempting to use to create the email.
is it going to get flagged as spam? Probably.
The preferred choice for implementing this service is to use email newsletter software, such as MailChimp or GetResponse. Steer people to the newsletter signup page, and make sure to double opt-in or you 'll be violating the CAN SPAM laws. https://en.wikipedia.org/wiki/CAN-SPAM_Act_of_2003
I’t to reply to 500 people which is hectic by using horde cpanel webmail
Based on the content of your posts, I suggest you hire someone else to write the emails.
An alternative option to MailChimp is SendGrid. SendGrid provides a nice API to deploy mass email marketing campaigns in a safe and reliable manor.
This may provide some insight on host limits although I would question the reliability.