I have a client complaining that on occasion email they send is taking up to 4 days to process. What would cause this issue? The client is on mail1. The latest occurrence happened over the weekend. Email they sent on Thursday, 12/14/06, didn’t arrive until today, 12/18/06. They have also complained that mail sometimes never makes it to the recipient.
We have occasionally had some mail delays, but nothing like that 
Only thing I can think that may have caused such an issue is possibly a greylisting. I know Yahoo! is greylisting on their free services, and it is causing some large delays, but it is no fault of the sending server.
I spoke with my client about the issue. They are not sending email to Yahoo accounts. However, I did discover that the delays are only happening when they send email to sms. I suppose it’s very likely the issue is with their wireless provider.
And to clarify, there have been no issues on mail1 since 12/14/06? e.g. down for maintenance
How long does it take for an email sitting in queue on a JH mail server to be deemed undeliverable?
I will have to defer this to Deepak or Tanmaya, as I can’t tell you with a honest manner.
Mai11 has not had any issues, and that issue was very minor for a queue cleanup.
It depends mostly on the availability of destination server.
Greylisting can delay introduce big delays unless you are mailing regularly in a day to the destination IP.
retry interval can be 30 mins, and more in case of multiple deferrels.
Deepak also confirmed in a support ticket that mail sits in queue for 24 hrs until it is deemed undeliverable. It then is sent back to the sender as such.
Thanks