The search features in AltaMail are easy to use, but also allow a very powerful range of search options if needed.

For a simple search, tap on the magnifying glass button in the top of the Inbox. Type some text in the search field and the text will be matched with all details of an email, eg "From", "Subject" etc.
Your previous searches will be displayed in a list. You can delete previous searches by swiping left over the search text.

You can change simple search behaviour in the App Settings - Email Section. (View) These settings are 'Search as you type' and 'Closing search clears text'.

You can also tap on the global search button (circle) to the right of the search field. This will search in all email folders in all accounts. This search may take some time to complete due to the number of emails.

For an advanced search, tap on the "More/..." button on the left of the search field to open the advanced search screen.
You can also edit advanced searches in the left side panel - Smart Folders - Email Searches, and tap on the cog button.

In the advanced search screen, you can add multiple search fields with the "+" button below the Search section. Choose the search field and then you can enter text for the search, or change the Yes/No value button if that applies.
When adding multiple fields, you can require that all fields match (and) or any fields match (or) by tapping on the and/or button between fields.

Tap on "Search" to start a search in a new screen. If your email account supports it, you can use "Search Server" which will send the search to your email server. This email server search option may take some time and has less field options than an AltaMail search.
The "Global Search" will search all accounts and folders. This may cause a delay as all folders will need to be loaded and will use quite a bit of memory. Also, if you refresh the global search screen you could potentially refresh every folder and email account if you confirm the refresh.

You can save a search for use later using the action menu button in the top right. Using the action menu, you can also choose a saved search or copy the search and paste it into another saved search screen.

You can use Nested Searches (View) for more complex searches with mixed fields of "and" and "or".