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The main thing that ESPs do that I've found beneficial is the work of SMTP relay services. There are so many different and evolving processes to minimize spam, that the biggest job related to email in my office was making sure it actually got to the recipient address. What the ESPs do that most companies do not do (and generally cannot do) is work with ISPs to monitor bounce rates and make sure that accidental blacklisting doesn't happen. In the end, I've found it to be cheaper to pay an ESP than to maintain our own in-house SMTP server.