check the partitions that will be used if you archive the emails, plus check the log files.
If every sent mail will add an entry into a log file, the fill will grow up quickly.
Morover check that you have an A record + an MX record, which designate the public ip of the server
ideally check the reverse DNS also, some ISP make a reverse dns lookup in order to allow or not the sender.
Actually, take care of the gerylisting, it's a recent feature: ISP's block emails for few minutes in order to prevent spamming, non-spammer system are aware of greylisting (ie postifx) and few minutes later the email is send again (spammer don't care about the fact that the mail is blocked or not). If your system does not handle this, then you will have a high failure rate
Because you will need to see what goes wrong if an email is not sent