You do not need a different app, a web interface, or anything installed.

• Open the forwarded message in whatever mail app you normally use.
• Press reply and write as usual.
• Send it.

Your reply leaves with the alias as the sender. The person on the other end sees the same address they wrote to, and your own address never appears in the headers.

How it works, briefly

The forwarded message arrives from a per conversation address that belongs to your alias. Replying to it tells our relay which alias to send as and who to send to. Nothing about your real address is in the message that leaves.

If the reply did not arrive

• The alias is switched off. An alias that is off cannot send either.
• You replied from an address that is not one of your verified recipients. Only your own addresses can send as your aliases, otherwise anybody who saw a forwarded message could send as you.
• The far end refused it, in which case you get a bounce back. Reading a bounce covers what it means.

Starting a conversation instead of replying

Writing to somebody who has never written to you is a different action with a different rule. See Writing first, as an alias.