Who actually receives my emailing?

Short answer: Only contacts who have an email address and have given their marketing consent. The others are dropped automatically, and the send window tells you how many before you confirm.

Under the recipient list, two lines answer each other:

That count is worked out before the send, so you see the message's real reach before confirming it.

The window's summary names only two, but a send can drop a contact for three.

No primary email address. The check is not about having an address at all, but about having one marked primary. A contact can therefore hold an address on their detail sheet and still be dropped, if none has been set as the primary one. The contact recorded with only a phone number exists too, but is rarer.

No marketing consent. Consent is given channel by channel, and it is the primary address's consent that counts. Without it you cannot send that person a promotional message: this is a legal obligation, not a convenience setting.

Address known to be invalid. An address that verification has found undeliverable is dropped before the send — a typo, a domain that does not exist, a closed mailbox. The “Verify emails” button in the action bar runs that verification on the selected contacts, which lets you spot those addresses before composing rather than after.

There is nothing for you to do: DAGOBA automatically adds a notice carrying the unsubscribe link to every one of your emailings. There is no field to insert and no box to tick — compliance is handled for you.

A contact who unsubscribes drops out of your later sends without you having to do anything: they will then show up among the dropped contacts, under the marketing-consent reason.

The per-recipient detail, including the reason a contact was dropped, is in the history. See Where can I see the emails I have sent.