You can set up iOS Mail app to allow you to send from any of your alias addresses, including those at your own domain.
Please note: Due to a limitation in iOS, aliases can only be added to the iOS Mail app if the account is set up manually.
- Open your device's Settings.
- Tap Mail. This is Passwords & Accounts for iOS 13, and Mail, Contacts, Calendars in iOS 9 or older versions.
- Tap your Fastmail account in the Accounts list. It will be labelled with whatever you typed in for the Description field when you set it up.
- Tap Account, then tap in the Email field.
- Tap Add Another Email… and type in the email address you wish to use then hit return on the keyboard. Repeat for all the email addresses you want to use and tap Done.
Now, when you compose a new message in Mail, you can tap the Cc/Bcc, From field and choose from any of the email addresses you have added.