Yes, this is valid and they can ask documents via this mode.
As per Section 169(1)(c) of CGST act 2017, Any decision, order, summons, notice or other communication under this Act or the rules made thereunder shall be served inter alia by sending a communication to his e-mail address provided at the time of registration or as amended from time to time.
However, as per Circular 122/41/2019-GST, CBIC has implemented a system for electronic (digital) generation of a Document Identification Number (DIN) for all communications sent by its offices to taxpayers and other concerned persons.
And Any communication issued without an electronically generated DIN shall be regularized within 15 working days of its issuance, by:
(i) obtaining the post facto approval of the immediate superior officer as regards the justification of issuing the communication without the electronically generated DIN;
(ii) mandatorily electronically generating the DIN after post facto approval; and
(iii) printing the electronically generated pro-forma bearing the DIN and filing it in the concerned file.
Therefore, in given case E-mail issued without DIN need to be regularized within 15 working days and after that period if it is regularized as above then that E-mail becomes valid, however if it is not regularized then that E-mail will remain invalid.