OutlookAddIn_getMetaData.ts

  private getMetadata() {
    this.state.graphController.retrieveMailMetadata(this.props.mail.id)
      .then((response) => {
        if (response !== null) {
          this.setState((prevState: IOutlook2SharePointState, props: IOutlook2SharePointProps) => {
            return {
              mailMetadata: response
            };
          });
        }
      });
  }

...

export default class GraphController {
  private client: MSGraphClient;
  private metadataExtensionName = 'mmsharepoint.onmicrosoft.MailStorage';

  public retrieveMailMetadata(mailId: string): Promise<any> {
    const apiUrl = `/me/messages/${mailId}`;
    const expand = `Extensions($filter=id eq 'Microsoft.OutlookServices.OpenTypeExtension.${this.metadataExtensionName}')`;
    return this.client
            .api(apiUrl)
            .version('v1.0')
            .expand(expand)
            .select('id,subject,extensions')
            .get()
            .then((response) => {
              if (typeof response.extensions !== 'undefined' && response.extensions !== null) {
                const metadata: IMailMetadata = {
                  extensionName: response.extensions[0].extensionName,
                  saveDisplayName: response.extensions[0].saveDisplayName,
                  saveUrl: response.extensions[0].saveUrl,
                  savedDate: new Date(response.extensions[0].savedDate)
                };
                return metadata; 
              }
              else {
                return null;
              }               
            },
            (error) => {
              console.log(error);
              return null;
            });

  }
}