I filed/sent my DC State tax via mail quite some time ago but it hasn't been accepted yet, what should I do to make sure it went through?

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If you did not use a mailing service such as certified mail or UPS to track your mailed return, you can try contacting them to see if it has been received,  In the future, whenever you mail a tax return or a tax payment, use a service with tracking.

https://otr.cfo.dc.gov/service/customer-service-center