Over 100 recipients in an email

The maximum number of recipients per message for iCloud Mail is 500 recipients (Mailbox size and message sending limits in iCloud - Apple Support). Are you using an @iCloud.com or similar email address through the Mail app, or connecting through an external email provider (Gmail, etc)? If you're using an external email provider and/or their app, they might have other limits imposed.

applesuoportTLC-0770 Said:

"Over 100 recipients in an email: How can I send an email to more than 100 addresses?"

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What is the Domain for this eMail Address?

Contact the eMail provider, asking them if this is an option on their site directly (rather then from the mail app).

If iCloud:

Recommend this to Apple:

Do your Part: Provide Apple with feedback on this, recommending that they add more recipients.  Apple may not get back to you directly, but the more feedback they receive on this, the more they will know what three customers prefer and what to include in updates to come.

1. Go Here: Feedback - iCloud - Apple
2. Select: "iCloud Mail" for the "Feedback Type"
3. Comment: that you want to recommend more storage for one account.
4. Select: "Mail" for "Which applications do you use with iCloud?"
5. Proceed from there as necessary