Two limits apply to every paid account
Pro allows up to 20 outgoing messages per account and up to three per address in a rolling 24-hour period. Ultra allows up to 50 per account and up to five per address over the same period. Free is receiving-only and does not include outgoing mail.
The account and address allowances apply together. Creating more addresses does not increase the account allowance, and giving someone access to a mailbox does not multiply that mailbox's allowance. All supported types of outgoing message use the same limits rather than separate pools.
Rolling periods do not reset at midnight
A rolling 24-hour limit looks back over the preceding 24 hours at the time of a send. It is not a daily counter that resets at midnight in your time zone. Earlier sends leave the window as they become more than 24 hours old.
There is also at least a 60-second pause between sends and a limit of 10 outgoing messages per account per hour. These shorter limits still apply when a 24-hour allowance remains. Use the app's remaining allowance and status instead of assuming that the local date gives you a fresh quota.
Personal correspondence, not bulk sending
A message has one recipient. Additional recipient, domain and service-wide safeguards can restrict sending before the plan maximum is reached. Some failed submission attempts also consume allowance, because repeatedly retrying a blocked send must not bypass anti-spam controls.
Check Sent and any displayed error before trying again. A submitted message is not a guarantee that the recipient's provider will place it in an inbox. Maily is not a mailing-list or campaign platform: spam, harassment and attempts to evade safeguards are not permitted.