Tips for knowing the authenticity of an offer letter

Hi Anitha,

I am unsure if your advice is free, if not please ignore my request.

I am in South Africa at the moment and applied for a Legal Advisory job. I received an offer from an LLC without an interview, and the offer letter also does not contain the landline number 9714...; instead, it is 9715...

The first offer mail was sent from a Gmail account. I questioned this with the company by sending them a mail to the address on their website being [-----.com] and asked if the offer was legitimate.

I then received a response from this e-mail that they are awaiting my signed offer. How will I know now if the offer is legitimate?