Check out the NZNOG mailing list's archives - this was mentioned as an issue recently (specificly xtra -> aol).
On 07/06/06, Nick Rout wrote:
A friend uses xtra as his ISP. His mother has an aol.com account in the UK. About 50% of the time he is getting his emails to her bounced by xtra's smtp server claiming not to have delivered them after 4 days. The rest of the time they appear to go through.
