This sort of thing is becoming a common issue. In an effort to curtail spam, providers are getting drastic with the blocking. Some places (such as AOL) will block the IP address if too many E-mails come in under an hour. That makes a big problem for places like DH and your own ISP who may be sending a lot of E-mails out.
DH has been extremely good about working to get removed from spam lists, so be sure to let them know when ever you experience a spam listing like this.
Also, if you need to send out an E-mail your pop3/IMAP and webmail use different IP addresses, so if one doesn't work you may well be able to use the other.
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