How do I keep boundaries with colleagues of the opposite sex who are not mahram, to protect my marriage?

Keeping boundaries with non-mahram is a fence that protects the marriage before problems begin, not a sign of suspicion. Keep work matters to work: avoid deep personal conversations, closed one-to-one meetings, and sharing marital problems with non-mahram colleagues. Watch your tone and messages so they do not open the door to feelings. If someone starts to feel special in your heart, that is a signal to add distance, not to draw closer. Invest that closeness in your own spouse. Being professional and polite does not conflict with guarding your dignity. Boundaries kept early save a lot of regret. See everyday adab.