Is it legal to notify clients by mail or email that you have left a company?

Generally speaking, an employee may let clients of his/her company know that he or she has left the company's employment, is working someplace new, has his/her own business, etc. That's the point of a non-compete or nonsolitication--to put a limit on the ability which employees otherwise have, to try to "poach" clients or compete with their former employer.

Of course, and here's a potential kicker, employees may not use confidential business information generally to compete, and very specifically if there is (and to the extent of) a confidentiality agreement. So if the confidentiality agreement says that customer lists, etc. are confidential business information and may not be used by employees for their own benefit, a former employee could not mail to customers of that business--that would be using the confidential information for the employee's benefit.

There would be ways around that--generally speaking, if "confidential" information is known through other means, it's no longer confidiential. So an employee could mail to all potential customers in a given industry or geographic region, and if that includes former clients, that's not a violation of confidentiality (though the former clients should get the same materials as non-clients). Or clients that the employee knows through other channels (e.g. an industry group) or knew before joining the company should also be fair game...though read the confidentiality agreement and make sure you comply.