ILIAS does not have such an option. Mails are always sent immediately by the application. However, ILIAS does not send the mails itself. It relies on a Mail Transfer Agent (MTA) to do that (sendmail, postfix, etc.). Alternatively, ILIAS may also use a SMTP Relay Host. You could try to enforce certain limits at this level. I would not worry too much though, unless you experience actual problems with graylisting (or even blacklisting) on certain mail servers.
You mentioned Sendmail, which I don't know very well. But if you consider using Postfix as your MTA, you may want to have a look at the options "smtp_destination_concurrency_limit" and "smtp_destination_rate_delay" to reduce the number of mails relayed to the same destination per timeframe.
You mentioned Sendmail, which I don't know very well. But if you consider using Postfix as your MTA, you may want to have a look at the options "smtp_destination_concurrency_limit" and "smtp_destination_rate_delay" to reduce the number of mails relayed to the same destination per timeframe.