I can connect to my mailbox but my terminal returns an SMTP error when I try to send any email

Check that your terminal is using the correct SMTP parameters (Gandi's SMTP server address, port number and SSL/TLS security). Remember that you must authenticate yourself on our SMTP server using a username (= your Gandi mailbox's full address) and a password (= your Gandi mailbox's password). A common mistake is to forget to activate authentication for outgoing mail.

See Gandi's SMTP parameters to configure your terminal.
Resetting a Gandi mailbox password