Do you see the sent messages in "Sent" folder via your web-interface for your e-mail provider?
If not, - you may need to configure what to do with the "Sent" messages.
If yes, and it is only Aquamail where you don't see "sent" messages, there could be a few different reasons (usually unrelated to Aquamail), depending on how your mail server is set up. One of the most likely culprits is that you might have multiple folders for "Sent" messages, as different servers and different e-mail client desktop programs and mobile apps might create different folders for those.
So, if your Aquamail is set to sync with one that exists but is not the one that the server considers as the default "Sent" folder, you wouldn't see anything.
In English, most typical names that I've seen for the sent folders are: "Sent", "Sent messages", "sent-mail".
If you have a different language (especially with Gmail), the name can be yet different.
So, I'd suggest to browse through all folders available on the server as seen by Aquamail and checking which one should your [Sent] be synced with.
Read the 4th item in the FAQ:
http://www.aqua-mail.com/?page_id=227If these suggestions did not help, please tell us more: what is your e-mail provider (Gmail? ...?)
Can you see sent messages via the web interface?
Can you see sent messages in [Sent] in Aquamail when you send a messages from Aquamail?