Unpopular opinion, but... set up your own e-mail server. I've been running my own off my home connection for quite a while with basically no spam problems.
Mails sent from dialup IPs are almost guaranteed to be classified as spam (probably to prevent spam sent by malware). Many servers won't even accept such mail at all.
Wanted to comment the same. Initial setup is a bit bumpy - make sure reverse dns is ok, set up dkim, dmarc, spf - but once up, is should be fine, especially if it's only to send, and not receive.
Just an addition to that:
Don't use the cheapest possible provider available if you get a VPS to use as an email server; chances are the spammers are doing the same thing and that your emails will be sent straight to spam.
DigitalOcean, OVH and Hetzner are especially difficult. It is possible to run an email server there, but getting the IP reputation up will be a tedious process and some providers will simply reject sent emails simply for using one of those hosts.
Thank you for sharing your experience on digitalocean and OVH hosting providers. I want to understand the issue a bit more because we have a plan to host email service on both the platforms. Your inputs will help us to decide to go with them or to choose any other service provider. Can you please let me know when you sent those emails using both service providers is your emails authenticated with SPF and DKIM or you have used any free email service provider like gmail or yahoo as a from email address.
Thank you in advance.
I've had luck with digital ocean by sending friends emails (who have accounts at major providers) and having them mark it as not spam a few times over the course of a week or two. After that, it worked without issue.
It is definitely possible to get it up and running, but there's often problems due to entire ranges having bad reputation. There was a gigantic thread about it in the Mailop mailing list last year. If you have access to it's archives then it's definitely a worthwhile read (and entertaining too!).