The receiving of emails is handled by different policies, determinations, rules and filters by each email provider. These are partially obtained from other sources and classify emails as "undesirable" or "spam".
Possible reasons for the detection of "spam mails" are:
Many providers use procedures for email authentication, which confirms the identity of the sender.
The authentication data of the messages will be checked there. These data should be in the header sections "signed-by"- or "mailed-by", which can be found in the message details below the subject line. If the sender hasn't make a statement, it is not securely establish if the message is authentic.
Manually moved messages in the spam folder:
If messages of a specific sender are repeatedly detected as spam, the behavior of the recipient is remembered by the system and moves the messages of this sender automatically in the spam folder.
Message content is empty
Emails without subject and content are classified as spam, too.
Whitelist - Blacklist
Receiver can set specific emails on the "Whitelist". In this list emails will never be marked as spam. If emails were set on the "Blacklist", these always will be marked as spam.
If you have no success after the consideration of possible reasons, please contact the email provider of the recipient. If you don't find a solution by the email provider, please contact the customer service center and specify the necessary data for authentication: customer ID, telephone passwort and contract number. Please specify also the subject line of the email, which is marked as spam.