The matched address will be shown as via in the addresses section of received messages (between the message header and message text).
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 identity settings to match the username of an email address (the part before the @ sign).
+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.
If you want to match a catch-all email address, this regex is usually fine, provided all usernames for the domain are yours:
.*