use of bcc

The "bcc" (blind carbon copy) field is a copy of an email message sent to someone whose email address does not appear in the message so that other receipts can not see his address. So you are able to write 5 persons at the same time by using bcc and no one else would know to who else you sent it.

-> that also means that you can leave the "to" field empty

I quote:

Limits on Sending Messages

iCloud has several safeguards to ensure that only iCloud members can send messages using the iCloud outgoing mail server. Among these safeguards are reasonable limitations on:

iCloud email has been designed primarily for personal use. Sending unsolicited bulk email messages through iCloud email servers is prohibited. To review the iCloud membership agreement and acceptable use policy, please see the iCloud Terms and Conditions.

So to send to 160 recipients you will have to split them into two sendings. The limitations on the Bcc field will be the same as on the others, as above.

Ferry Horváth wrote:

In the [bcc] I can put also 100 recipients instead of the 5 David is mentioning?

I've never tried, but I don't see why not. I imagine David simply chose 5 as an example.

Sorry for the confusion... Yes as Roger said, the "5" recipients were only an example for explaining that no one else (e.g. the other 4 recipients in this case) would not see the email addresses of the other recipients when using "bcc".