Can't Send Email with Trojita

[Johnny Blue]

Set up an IMAP account, using IMAP and SMTP details that work perfectly in other Linux OSs, but with Porteus/Trojita I get an error message box:

Failed to Send Mail

APPEND failed: Invalid mailbox name.

I really like using Porteus, fast, good on the eye, can read Office documents if I load the relevant module, so all is good, apart from this sending email problem (receiving/reading emails is fine, BTW).

Have searched high and low on the internet for a solution, but the combination of Porteus, LXQT and Plusnet (my ISP) is pretty unique, it would seem...