Learn in the textbookHow Email Works and Specifying Recipients
DefinitionA mailbox (メールボックス) is the inbox on a mail server where delivered email is kept for each user.
ExplanationMail does not go straight to the recipient's device; it first enters a storage area on the recipient's mail server. The recipient can connect and read it at any convenient time, so sending and receiving need not happen at once. Reading and writing that storage area from a browser is called webmail (Web メール).
AnalogyThink of a parcel locker in an apartment building. It holds packages while you are out, and you collect them when you get home.