how to avoid my sent email not be classified as spam? [closed]

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I am operating a website that can send invitation to other people, I encountered a problem that the emails sent from my website are all classified as spam in the major email sevice providers, I am just wondering how I can avoid this as much as possible.

Basically what I am doing now:

1) User input friend's email

2) I use python to send the email, the email is simple - pure text, just a line of words to tell people that your friend is inviting you, and the website has created an account for him, the password开发者_如何学Python is **

3) The 'from' of the email is the user who is sending the invitation(I am wondering if this contribute to the spamness?), not my website's email.

Thanks. Bin

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1. How to send emails and avoid them being classified as spam will give you some useful tips.
2. Ways to prevent email being blocked as Spam

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Yes. Your third point is a major reason you get marked as spam. You are sending emails with addresses from another domain then the domain of your server, and you change which email this is all the time. This is a typical spammers behavior.