Dumb example: you might have published a paper while working at a company, but years later the company went bankrupt and ceased to operate. Now somebody else is owning the domains and they will not make you the favour to give you an email address.
Notable example: Sun Microsystems. But there are many more, of course.
Or you just moved from one university to another. Or you published while on grad school and then moved somewhere else.
Dumb example: you might have published a paper while working at a company, but years later the company went bankrupt and ceased to operate. Now somebody else is owning the domains and they will not make you the favour to give you an email address.
Notable example: Sun Microsystems. But there are many more, of course.
Or you just moved from one university to another. Or you published while on grad school and then moved somewhere else.