Unless you can find exactly why the e-mail is classified as spam to do changes on the mail server used the only option is to do it on the gmail side.
I see two options, one is to select those messages on gmail and classify them as no spam, after some time gmail will learn that the messages are legit.
The second option is to create a filter to match those messages and set the filter to never send those messages to spam.
This google article could help https://support.google.com/mail/answer/9008?hl=en
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