If you don't want to interrupt people or wish to wait to send an email, you can use "Send after". This will schedule an email to be sent later at a time you choose.

If you have not already done so, tap on the "..." button in the top of the new email screen to show all features and tool buttons. Once you have completed a new email then you can choose the time to send the email.
On an iPad, tap on the clock button at the top to choose a time. On an iPhone tap on the action menu button (circle with 3 dots icon) and then select "Send after time".
Your email should now be sent at this time even when AltaMail is not open, unless there are issues like network access etc. Make sure background fetch is turned on for AltaMail in the Apple device Settings app.
Note: Do not force quit AltaMail or notifications and send after actions can not be completed.

You can automatically delay sending emails until office hours using 'Restrictions for sending email' (View)

You can adjust the default hour of day in the app settings - Email Management. Tap on "Email Reminder Settings" to show the time settings. Then you can adjust "Start of working day".