The Gmail system considers a number of factors when classifying messages as spam.
Google's Spam filters are designed to respond to user preferences at scale. Please note that our spam engine learns over time and results are often not immediate.
However, there are certain steps you could take to make it less likely that these messages will be marked as spam and to also improve a sending domain’s spam reputation over time.
The sender having a valid SPF record is especially important. Please see https://support.google.com/a/answer/33786.