Any errors or status information from the destination server will be passed back to the sending MTA through the proxy.
[3] SMTP proxies often implement the first and/or only layer of defence in an inbound anti-spam filtering system, where they can analyze messages using a spam content filter or antivirus program, block or rate limit connections using DNS blacklists and reputation systems, and load-balance SMTP connections to prevent overloading of mail servers.
When slowed down, some malicious sources of SMTP traffic such as spambots tend to give up rather than continuing to deliver a full email message.
[4] Network tarpitting can be challenging to implement within an email server, since each SMTP connection is processed more slowly than normal, often holding up precious system resources such as memory and CPU.
SMTP proxies are sometimes inserted between sending mail servers on a local network, and their receiving counterparts on the Internet.