sending limit in Apple Mail

Is this an icloud email account ? If it is then there is a limit to how many messages you can send in a day, and the number of recipients for an email : Mailbox size and message sending limits in iCloud - Apple Support :

There are several safeguards in place to make sure that only iCloud members can send messages using iCloud. One of these safeguards includes reasonable limits on:

• The total number of messages you can send each day (200 messages)
• The number of recipients you can message each day (1000 recipients)
• The maximum number of recipients per message (100 recipients)
• The size of incoming and outgoing messages (20 MB, up to 5 GB with Mail Drop turned on)

If it's not an iCloud account then check with your email provider to see if they have any limits.

You can be using other email accounts with the Mail app (e.g. a gmail account, hotmail, yahoo etc), it doesn't have to have an icloud email address.

You said in your first post that the 'message that must have had a hundred addressees' - there is a limit of 100 recipients for a message, if it had over 100 then try sending it as two separate emails with less than 100 on each.

If you no longer trying to send that message and you aren't breaking any of the other limits with what you are trying to send then it should work. From the link un my first reply :

If you continue to exceed message sending limits, follow these steps:

• Check your Mail Outbox to see if it contains unsent messages. Resend or delete the unsent messages.
• Check for any network monitoring software that might be using your iCloud account to automatically send messages (like firewalls or internet security software that sends alerts about security issues).
• If you send messages to groups, make sure all email addresses in the group are valid (For example, make sure that all email addresses are spelled correctly).