In case you don't know, Entourage has not been "definitively" compatible with El Capitan, Sierra, or High Sierra, and Microsoft is no longer supporting it. (I use Outlook 2016 as part of Office 2016 for my EMail client).
Here are the results of a google search I just did for "Entourage and sending an EMail to more than 25 recipients":
When I did it previously, one of those links mentioned about checking with your ISP. Also, some (maybe only 1 or 2) of those links mention something about the "smtp" setting for your outgoing (ie, sending) mail. I wish I could help you in a more definitive fashion, but I have no experience with that issue.
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