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100 mails sounds like a "human" limit, and I would not be supprised to find out this is something your host is resposible for, as they are afraide of you perhaps sending out spam.
Many times they simply have a limit of say, max. 100 mails every 15 minutes.
Other times they analyse the headers of the outgoing post, and desides what kind of limiting they will enforce.
As the mails send from the XOOPS system does have incomplete headers, both senarios are likely. But you could ask your host about their politics.
And for the XOOPS mailer system... perhaps it's time to start thinking about some scheduling system when sending out mails?
Best regards,
Daigoro