An SMTP SmartHost is simply an external SMTP server which allows email to be relayed through it. You might want this as an 'add on' to your existing in house SMTP server to filter out certain traffic and direct it explicitly - and only it - vi an external sever. This might be due to geo blocks or the need to control traffic more tightly than normal.
There are numerous commercial companies that offer this type of service, with many allowing a number - perhaps a few hundred emails a day - to be sent free. But... almost al of them require the sender to be registered in some way with the Smarthost supplier, and not just rely on a username and password. This is fine if all the traffic comes from one or even a few Domains, but not practical for relaying general Internet traffic which will come from many, unknown, users.
The only company who handles this situation we have come across (purely as a client) is Prolateral who are based in the UK not far from London. They offer a free 10 emails a day for free and 1,000 a day would cost less than £10/ $13 per month. To use it all one needs to do is use their SMTP server, and an agreed username and password. Works fine as a Smarthost and also as a simple external SMTP server.
Other vendors in the same space include:
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