The display of [SPAM] in the subject lines of some campaigns is being added by the receiving server's spam filter. Typically this occurs due to something the receiving server did not like about the campaign.
The IT team at the receiving domain should be able to explain why it's being considered SPAM. This tends to be more frequent with private domains since they have custom spam filters.
Our system has no way to foresee to prevent that from happening but we offer Authentication options to assist in delivery of campaigns to your recipients. Contact your CSM or the Support team for more information about Authentication options.