In the account list:
Long press the account -> Account setup -> Manual -> Next.
It would help if you posted your current outgoing server settings, and let me know if the device is on WiFi or mobile when the issue occurs.
Obviously I'm not asking for your login / password, just server name / encryption type / port number.
Note that AquaMail won't let you complete account setup until it passes server tests (the app connects, logs in, tries to send a message and aborts). This means that something else changed - the device's connectivity / network, or maybe the server itself now requires different settings.