A great many small businesses still trade from an email address that belongs to somebody else. It might be the one handed over by the internet provider when the line was installed, or a free mailbox opened quickly in the first week of trading because something was needed straight away and it worked. Either way, the address itself is borrowed rather than owned, and that distinction stays invisible for years because nothing about it looks temporary. Customers write to it, invoices carry it, it goes on the van and the business cards and the sign, and it quietly becomes the single route by which the outside world reaches the business. The arrangement is fine until the day it is not, and the day it is not tends to arrive with no warning at all.
What makes it fragile is that the address is attached to a relationship the business may one day want to end, or which may end on its own. Change internet provider - because the service was poor, or because the business moved premises, or because a better price appeared - and an address tied to the old provider can go with them. A free mailbox is not tied to a supplier, but it is still somebody else's system, subject to somebody else's rules, and it is not something the business can point at and say it owns. In each case the business does not merely lose a mailbox; it loses the address that every past customer, supplier and directory has on file, and there is no way to take that address along.
The alternative is unglamorous and cheap: register a domain name for the business and use email on that domain. The important property is not that it looks more professional, though it does, and customers do read a proper address as a sign that a business is established. The important property is that the domain is the business's own. Providers can be changed, mail can be moved from one service to another, the website can be rebuilt three times, and the address stays exactly the same throughout, because the part that customers memorise is the part the business controls. It converts the address from something rented into something owned, and the cost of doing so is roughly the price of a few cups of coffee a year.
The practical move for a business currently on a borrowed address is worth making sooner rather than later, and it does not have to be abrupt. Register the domain, set up the new addresses, and run both for a period while the new one is put on everything the business sends out and everything it is listed in; mail to the old address can usually be forwarded during the changeover so nothing is lost while the word spreads. It is a modest afternoon's work at a quiet time, and it is one of those jobs that is straightforward while everything is calm and genuinely difficult in the week a provider is being changed under pressure. The reason to do it is simple: the address customers use to reach the business should not be something the business can lose by changing suppliers.