Should I deposit the check? Yes ...please do.
How should I get an explanation for this? Did you review your original and amended return carefully ? Does that $600 show up somewhere ? Did you check/review your tax transcripts against those returns yet ? Doing so may answer your question. https://www.irs.gov/payments/your-online-account
Should I try writing to them by conventional mail? You can however expect a response to take months.
