When teaching correspondence courses, good rapport with your students is important. A timely return of lessons, especially the first few, and positive comments help achieve this rapport. Also, a friendly comment on the first lesson can help alleviate the isolation felt by students. Assessing Student's Work Notes reflecting your assessment of the student's work are essential. A lesson should never be returned without some indication of the quality of the work, especially if it is lacking in some respect.
Straightforward comments are best.
Sarcasm, although sometimes tempting, appears more harsh than it is usually meant and frequently is discouraging rather than motivating. Negative comments that offer no help are worse than useless. In all cases, students must be able to tell exactly what is being counted wrong. If you are grading on a point system, indicate the number of points taken off for each mistake. Include some word as to what is right also.