If you change your e-mail provider, you usually forget the old address sooner or later. There are at least two reasons why you should regularly log into the old account.
Anyone who has a new e-mail address should log in regularly in the old account. Many vendors deactivate the e-mail address if they do not log in to the account at regular intervals – and then others cannot even accidentally write to the old address and reach one.
Protection against misuse of the e-mail account
One recent example is Microsoft, which is introducing a rule for its e-mail service, Outlook.com, in late August, which requires users to log in at least every two years to keep their account active.
Those who keep their old mail accounts active and do not delete them also protect themselves against the possible misuse of their retired e-mail addresses when they are re-assigned. This applies in particular to addresses with names or parts of names.
Reassignment of the e-mail address is possible
A new assignment is not uncommon after a grace period with some e-mail providers. But there are also providers, the addresses of accounts that have been disabled or deleted by the user, for security reasons in principle block and not reassigned.