Add Zoho email provider
I came to know through RackSpce support that they only allow 250 similar mails/day. Per them if our PHP script utilizes their SMTP to send more than 250 mails even though they are of different content, they will be considered spam. ZohoMail has higher limit and economical for our business need.
i have been using http://bulkresponse.com/smtp-relay.php for high volumes they have very cheap rates and have in house team for white listing if you ever get black listed.
Paco Lazo commented
Rackspace is pretty expensive compared to zoho that only bills 3 bucks per month (24 yearly ) for unlimited aliases and domains.
I was surprised how good is Zoho email hosting. I would be more than happy to use it for my applications hosted on Cloudways.
Use Mailgun for this purpose (also from RackSpace) http://www.mailgun.com/
For that kind of service is much better use transactional email service, not smtp with your email hosting, bc. you can bring your email or server ip very fast to blacklist.