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Navigate via a computer or laptop to your email server and there should be an option to highlight the email as not spam. This should train it to recognise its not spam.
Then they should drop into the in box folder and not the spam folder.
When you log onto your email from your PC, when you see the messages in spam, there should be a setting on your email server where you can add the senders address to your safe list.
When the address has been added, any further emails from the address should be sent to your inbox and there should be no further problems receiving the message on your phone.
As you've probably gathered, this is a function of the email service rather than the device. Spam filters work differently, but it's usually content rather than the sender that triggers it - unless you've previously marked a sender as a spammer, of course.