Offer the conversation rather than conducting it in writing.

Correspondence opens difficult matters well and resolves them badly.

Writing is excellent for the opening move: it puts the facts down accurately, gives the reader time to absorb them privately, and avoids the ambush of an unexpected call. It is poor for everything after that. Disagreement over text is slow, each reply hardens the position, and neither side can hear the other's tone. Long written exchanges tend to become about the exchange itself rather than about the student, and by the fourth reply people are defending sentences rather than solving anything.

So the first message should state the facts, state the next steps and offer to talk, with something concrete about how: a couple of windows when you can be reached, or an invitation to nominate a time. That converts a written dispute into a ten minute conversation and usually ends it. It also signals confidence, because a person willing to talk is a person who is not hiding behind a keyboard.

When a reply arrives that is upset or accusatory, resist the pull to answer point by point. A thorough written rebuttal feels satisfying and reliably makes things worse, because each answered point is an invitation to a counterpoint. Acknowledge that they are unhappy, correct anything factually wrong in one clean sentence, and move to the offer of a conversation. Involve your supervisor or the relevant colleague early rather than late, and follow whatever your institution's process requires.