Don't abandon that domain name
Email holds the keys to the kingdom. All your password resets go through email, and abandoning an old domain name makes it easy for attackers to re-register the old domain and get your stuff.
The problem is especially grave for law firms where partnerships form, dissolve, and merge often, security researcher Gabor Szathmari points out. A merger or acquisition typically involves either new branding for the new firm, with a new domain name to match, or the acquired firm dropping their old branding and domain name. Letting those old domains expire is dangerous.