Inspect new messages and replies separately
Open a new message and examine the selected account. Repeat by replying to a message received at another of your addresses. Depending on the arrangement, these actions may choose different senders. Do not assume an address selected once remains the choice for every subsequent message.
Display the complete address if only a name is visible. Two accounts can share the same display name. An appropriate signature is not proof of the correct sender either; it may have been inserted separately. Treat the name, address and signature as three related but distinct details to check.
Test with a controlled recipient
Send a short test to another agreed mailbox. Inspect the address shown there, then reply. Check which of your accounts receives the response. This also reveals a different reply address that may be missed when looking only at the sent copy.
Test the sending routes actually used, such as webmail and a phone. This is particularly useful after adding an alias or changing accounts. An alias accepting incoming messages does not confirm the intended outgoing identity in every application. Give each test a distinct subject so you can connect the received result with the application used.
Make the working environment clearer
Give accounts distinguishable labels where the application supports that. Assign signatures and the default selection deliberately. If personal and business accounts appear together, pay particular attention when replying from a combined view.
For infrequently used role addresses, keep a brief note explaining the correct selection. If a required address remains unavailable in one application, have the configuration reviewed. Do not work around it with a misleading display name. What matters is the address actually used and a reply route that has been tested and understood.
