What are SMTP relays?
It's very important to grasp the actual nature of SMTP relays, especially if you are an email marketer and need to send out a high number of messages via an SMTP server. So: simply put, an SMTP relay is the very act, done by an outgoing mail server, to deliver any email to another server.
A comparison with snail mail may be clarifying. When you send a letter, your local post office sends it to another office in the recipient's town: well, this is a "relay". So when SMTP servers speak to each other — like paper mail's post offices — to ensure a proper email delivery, they simply forward a message from A to B, from the outgoing mail server to recipient's one.
SMTP relays are essential to send out transactional emails, newsletters, and all other forms of bulk mailing: to deal with such an amount of messages and deliver them correctly, SMTP servers route them through a trusted third party.
There's a remark to be made, though: and it's about the open relay.
When we speak of open SMTP relays we normally intend an outgoing server that accepts messages from other domains without asking for authentication. In other words, your SMTP server can be used as a "bridge" from two different ones to communicate. The main problem with open relays is that they can be employed by spammers to send bulk unsolicited messages: if it happens to yours, your IP will be probably quickly blacklisted and you will never be able to send mass emails from it.
That is why many internet providers put a limit to the number of how many relays you can do per day: a fair solution for normal, peer-to-peer emails — but very bad for bulk email marketing.
So, to prevent such an unfortunate situation, you can switch to a professional SMTP service like turboSMTP, which relies only on secured IPs and always ask for user's credentials to work. This will ensure that nobody outside your company will access your SMTP, and will also maximize your deliverability.