If you have a domain on Cloudflare, there is a beta feature that allows you to set up Email Routing. However, Cloudflare provides email forwarding, not hosting. You cannot send your emails through Cloudflare.
While Cloudflare’s email routing service doesn’t provide a way to send emails, you can use a service which provides SMTP login credentials. Next, you need to update the SPF record injected by Email Routing to add a given provider’s SPF. For example, for Google:
Create a custom TXT record for SPF: It should look like this: v=spf1 [IP ADDRESS] include:_spf.google.com include:[DOMAIN] ~all
As a result, your email will be trusted by the recipient and won’t land in spam.
Here is a good tutorial on how to set up Gmail.
Important note is that this is a deprecated Google feature. At some point, they could disable this functionality.