Well, this usually happends, when you aren't authorised on the server, meaning, yu can't just send emails withut authorisations... How you do that, I don't knw... Our IT set the main server to open relay, meaning, that the messagess can be ONLY sent from THAT server (no abuse).
But I guess, that in your case that isn't and the multiple recipments are disallowed at the "almst" same time. Talk to Your IT. There is also possible to delay the postings for a second or so, but at the end the script may crash because of the exceeded execution time.
Tell your IT, that you want to COMPLETLY open just yur machine, or the server, that script runs on... Otherway, I can write some dalay scripts... This MAY help, but it's 99.9%, that it will crash because of the exceeded execution time...
Berst regards. zer0Kode