This may be of interest to AOL users; I spend a bit of time in France and have a French ISP (Tiscali). For some reason AOL have decided to block all e-mails from Tiscali.fr to people using AOL; I would be interested to know if the same thing happens in the UK as I was thinking of changing from my current UK ISP to Tiscali in the UK.
Not only AOL but other isp's have blocked rival isp's from sending mail to their members. The main reason is spam and if one isp say AOL reports a problem to another ISP say FREESERVE that a lot of spam mail is being received from that server then they are supposed to take action and try to prevent the spam from it's members. If the isp fails to take action then the mail addresses can be blocked from that isp. AOL blocks it's own members if they send too much mail in one session. Exemptions are granted for companies and people who apply to aol for these. This helps to stop people spamming.