You can use mail-tester.com Show archive.org snapshot to check your application's e-mails for issues that might cause e-mails to be classified as spam.

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We make sure that your target audience has the best possible experience with your digital product. You get:

• Design tailored to your audience
• Proven processes customized to your needs
• An expert team of experienced designers

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They provide a one-time e-mail addresses that you can use to sign up etc. You can then check for scoring results of SpamAssassin and other potential issues.
You don't need to hit 10/10. Something around 9/10 is perfectly fine.

Note:

• For password-protected staging sites you will get an error for links that can not be resolved. This is fine, simply check production once available.
• As long as e-mails are sent with a sender address of the application's domain, our operations team will have configured your servers and DNS records to cover the most obvious cases (SPF records, reverse DNS lookup, etc). However, your application should provide plain-text bodies for HTML e-mails. This can be done automatically.
• Free plan is limited to 3 e-mails per day.

Further reading

• ActionMailer: How to send a test mail directly from the console
• Debugging SPF records