Warum, erklärt die Info von der o.g. Seite:

In order to maintain the quality of Amazon EC2 addresses for sending email, we enforce default limits on the amount of email that can be sent from EC2 accounts. If you wish to send larger amounts of email from EC2, you can apply to have these limits removed from your account by filling out this form.

Additionally, if you intend to send email from EC2, we suggest you provision one or more Elastic IP addresses (EIPs) and provide those addresses to us as part of this form. Amazon Web Services works with ISPs and Internet anti-SPAM organizations (like Spamhaus) to identify these validated EIPs to reduce the chance of your email being flagged as SPAM.

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