I am intimately familiar with the workings of a particular ISP, and I can speak with some amount of authority as to what happens to your email on the servers between the sender and the recipient.
Any major ISP will be doing such brisk trade in email traffic that your email will exist on the servers for, at most, a few seconds, before it is sent off, and deleted. Moments later, the disk space that was used will be overwritten by the next series of emails passing through. On the receiving end, sendmail will receive the email, and dutifully place it in your POP3 mailbox, or pass it along to somebody else who will, and deletes it.
With 25 to 40Gb of mail passing through a site on a particular day, moving as fast as 1Mb/s at times, it can quickly become expensive and time consuming to even attempt to keep a copy of each email, or to back it up to tape. The case in which you might need to worry would be if the ISP were to occasionally back up the POP3 mailboxes, for easy recovery in case of catastrophic loss. However, if you retrieve your email on a regular basis, and do not leave it on the server, or have it forwarded to another location where either you or someone you trust controls the data, this should not be a problem.
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I am intimately familiar with the workings of a particular ISP, and I can speak with some amount of authority as to what happens to your email on the servers between the sender and the recipient.
Any major ISP will be doing such brisk trade in email traffic that your email will exist on the servers for, at most, a few seconds, before it is sent off, and deleted. Moments later, the disk space that was used will be overwritten by the next series of emails passing through. On the receiving end, sendmail will receive the email, and dutifully place it in your POP3 mailbox, or pass it along to somebody else who will, and deletes it.
With 25 to 40Gb of mail passing through a site on a particular day, moving as fast as 1Mb/s at times, it can quickly become expensive and time consuming to even attempt to keep a copy of each email, or to back it up to tape. The case in which you might need to worry would be if the ISP were to occasionally back up the POP3 mailboxes, for easy recovery in case of catastrophic loss. However, if you retrieve your email on a regular basis, and do not leave it on the server, or have it forwarded to another location where either you or someone you trust controls the data, this should not be a problem.