Cerberus Networks provides a free SMTP relay server for our customers, SMTP3.cerberusmail.co.uk. There are a few restrictions on the server designed to protect the service from abuse in order to provide uninterrupted service to all of our customers.
SMTP3 is running SpamAssassin, configured to block messages that are obviously spam. Your mail client or mail server will log an error explaining this if you inadvertently try to send a mail that contains a very large number of spam-like characteristics.
SMTP3 is also rate-limited to prevent virus outbreaks, for example, from getting the entire server blacklisted. The limits are high so that you are able to send legitimate marketing mails, but you will be rate-limited if you go over the limit which is set at 500 mails per hour, with a total of 3000 mails allowed over an 8 hour period.
SMTP3 will accept a maximum of 100 recipients per message, and a maximum message size of 25MB.