My company (based in Japan) has a subsidiary in the UK that is having a problem receiving mail in a timely fashion. It seems that some messages from Japan, not all, are inexplicably being delivered to them between 1-5 weeks late. Their ISP tells them this was a glitch due to mail server overload. This sounds like a lame excuse to me, but I'm not an e-mail server administrator.
My initial advice to this subsidiary was to either change ISPs or host their e-mail server in-house. Are there any other options to guarantee that e-mail is promptly delivered? What would you do?