One of the problems that's recentralized mail is spam.
Because of spam senders, especially spam originating from residential and dial-up IPs (well, mostly residential now), many large email providers use blacklists (literally: DUL -- dial-up lists) to block SMTP access from IPs within a known residential range. Many also work to whitelist specific lists of known legitimate email providers.
May enterprise IT organizations require explicit whitelisting of domains from which email is sent. I kid you not. And many of these organizations apparently have never heard of SPF/DKIM.
A third level of resource are reputation-based services. Cisco's Ironport / Senderbase is among one of the better of these (it indicates both good and bad reputations, as opposed to simple blacklisting or whitelisting, as well as volume variations from a given IP and "neighborhood" reputations of nearby IP addresses.
The result is that it can be painful to try getting your mail delivered through to large email service providers. I've had the most significant issues with Yahoo and Aol, though others are at times problematic. This affects both individuals trying to configure small residential servers and businesses / enterprises.
Ideally tools such as SPF and DKIM help, but at best these indicate that a given IP address is included in a policy framework or that header integrity is assured -- there's still no basis for assuming that a given email message is or isn't spam. Getting SPF and DKIM properly configured can also involve hoop-jumping, especially for non-technical users.
Because of spam senders, especially spam originating from residential and dial-up IPs (well, mostly residential now), many large email providers use blacklists (literally: DUL -- dial-up lists) to block SMTP access from IPs within a known residential range. Many also work to whitelist specific lists of known legitimate email providers.
May enterprise IT organizations require explicit whitelisting of domains from which email is sent. I kid you not. And many of these organizations apparently have never heard of SPF/DKIM.
A third level of resource are reputation-based services. Cisco's Ironport / Senderbase is among one of the better of these (it indicates both good and bad reputations, as opposed to simple blacklisting or whitelisting, as well as volume variations from a given IP and "neighborhood" reputations of nearby IP addresses.
The result is that it can be painful to try getting your mail delivered through to large email service providers. I've had the most significant issues with Yahoo and Aol, though others are at times problematic. This affects both individuals trying to configure small residential servers and businesses / enterprises.
Ideally tools such as SPF and DKIM help, but at best these indicate that a given IP address is included in a policy framework or that header integrity is assured -- there's still no basis for assuming that a given email message is or isn't spam. Getting SPF and DKIM properly configured can also involve hoop-jumping, especially for non-technical users.