There is a specific failure that costs small businesses real money and produces no evidence at all: the email that was sent, was never delivered to the inbox, and was never reported as undelivered. The quote that got no reply. The invoice the customer says they never received. The appointment confirmation nobody saw. From the sender's side these look exactly like being ignored, and the usual conclusion - that the customer was not interested, or is disorganised - is both convenient and frequently wrong. The message went to a spam folder, and neither party knows.
The underlying reason is that mail systems now have to be aggressive about forgery. Anyone can put any address in the from field of an email, so receiving systems no longer take it at face value; they check whether the sending server is one the domain's owner has authorised. That authorisation is published as a small set of records attached to the domain, and if they are missing, incomplete or wrong, the receiving system is left guessing about a message claiming to be from your business. Some of those guesses put it in the spam folder, and the rules have become steadily stricter over the past few years, which is why mail that used to arrive may suddenly stop.
The practical consequence is that these records need to exist and to be right, and in many small businesses nobody has ever looked at them. They are usually set up correctly when a mail provider is configured properly at the start, and they are usually wrong when mail has been moved between providers, when a second service is sending on the business's behalf - an invoicing system, a booking platform, a newsletter tool - or when the domain and the mail were set up by different people at different times. Adding a service that sends mail as your business without authorising it is a common cause, and it takes effect immediately and silently.
This is not something to diagnose by guesswork, and it is not something to check by sending yourself a test message - your own mail system trusts you and will let it through. There are free tools that inspect a domain's published records and report exactly what is missing, and a mail provider's support will usually confirm what should be there. It is a half-hour job for whoever looks after the domain, it is done once, and it does not need repeating unless something changes. The reason to do it is the shape of the failure rather than its size: no other marketing problem is quite so completely invisible, because the messages that vanish take the evidence with them.