Your envelope does not match the address. This is the main reason why emails are considered spam.
Background: the address of the envelope (in your case admin@drt01.dco.fusa.be ) is not actually displayed to the user. It is simply used by mail servers to return error messages. Historically, this is because they are more relevant to the administrator than to the user. However, today its de facto standard sets the envelope address to the same as the user's address. Spammers do not actually do this because they either do not have the ability or they do not want to receive all error messages when sending spam. Or they don’t want exploited users with a Trojan worm to know about their atrocities.
In php / sendmail, this function is called the f parameter. You can read all about this in the function documentation: http://php.net/manual/en/function.mail.php
Classes like phpmailer or libmail do it themselves, and probably the best idea is to use one of them, because they have been tested, well designed and responsive to changes in their development!
The next thing you could do is set the reverse DNS in the same domain as the sender address. If you do this, you really deserve trust, because mail can be mapped to a server and, therefore, to a provider / administrator, and can be easily blacklisted in spam networks. Therefore, if you are spamming with correctly configured reverse DNS, you would not do this for long. There is a built-in directive in arpa rulse that setting up such a record takes some time, so it cannot be exploited in any way.
This, however, is pretty advanced, and there can only be one reverse dns entry per ip address. Therefore, you cannot do this in a shared hosting environment. You must also contact your provider to configure them, but you can use Google for all of this.
Amount:
- correct the envelope address, it should be fine
- if you can, set the reverse dns record
PS: in fact, there are no rules on how and when to consider mail as spam, and the above technologies have proved their effectiveness only in practice, but there is absolutely no guarantee. Each mail provider can do what he wants ... (yahoo was one of those candidates who did their own thing and annoyed everyone all the time)
luck
The surrican
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