Note that identities need to be enabled to be able to be matched and that identities of other accounts will not be considered.
Matching will be done only once on receiving a message, so changing the configuration will not change existing messages. You could clear local messages by long pressing a folder in the folder list and synchronize the messages again, though.
It is possible to configure a regex in the advanced identity settings (Navigation menu > Settings > Manual setup and account options > Identities > tap the identity > Advanced) to match the username of an email address (the part before the @ sign).
-Note that the domain name (the parts after the @ sign) always needs to be equal to the domain name of the identity. Since version 1.1640 it is possible to match the full email address with a regex, which can be useful for matching alias domain names.
+Note that the domain name (the parts after the @ sign) always needs to be equal to the domain name of the identity. This also implies that the username will be copied from a received message only if it matches the domain name of the matched identity. Since version 1.1640 it is possible to match the full email address with a regex, which can be useful for matching alias domain names.
If you want to match a catch-all email address, this regex is usually fine, provided all usernames for the domain are yours:
.*The username of a message being replied to will be used as the default username when editing of usernames is enabled in the advanced identity settings.