It makes a difference for tax and liability reasons if an action is done by a natural person vs a legal entity.
How to make sure that actions are ascribed to the legal entity rather than one's natural person?
What are the requirements by law and best practices?
For e-mail, I guess a e-mail domain name with the name of the company would be advisable? Or an e-mail signature that is attached to every e-mail?
The question of whether a person was acting on their own behalf or that of a company would generally be a question of fact, so if such a case came to court it would be for both sides to present evidence and argue for their interpretation of it.
In most cases the context makes it clear. You mention having a company email domain and associated email signature; that is certainly good practice and would go a long way towards creating a presumption that you were acting for a company. Also signing yourself with your job title or role (e.g. "Joe Bloggs, Chief Bottlewasher") makes it clear that you are speaking in your role as an employee.
The content of the communication also matters; if you use your company email address to order goods from a supplier that the company has used before then the recipient can reasonably assume that you are ordering on behalf of the company and a court is likely to agree. OTOH if you use the company address to send libellous emails then the recipient would have a much higher bar to claim that this was the company view rather than a personal one.
