Not a DOI
The resolution service at https://doi.org resolves DOIs. You have attempted to resolve content/323/3/990/tab-e-letters, which is not a DOI.
content/323/3/990/tab-e-letters may be a Handle (DOIs are a special sort of Handle). If content/323/3/990/tab-e-letters is a valid Handle you should be able to resolve it at https://hdl.handle.net.
WHAT CAN I DO NEXT?
- If you believe this DOI is valid, you may report this error using the form here.
- You can try to search again from DOI.ORG homepage