Thanks Ken.
Not successful but maybe there's a problem with my setup? This setup worked fine with IMAP, however.
Log entries:
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: accepted connection
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: extractor_init(0x56266de8d9e0)
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: proxy_check_input()
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: read & parse request-line
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: read & parse headers
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: conn flags: 0 upgrade flags: 0 tls req: 0
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: http_auth: status=0 scheme='' creds='Basic <response>'
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: http_auth: find client scheme
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: http_auth: found matching scheme: Basic
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: proxy_auth: authzid='15554441001@xxxxxxxxxxxx'
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: badlogin: pool-10-12-92-29.iot1.com [10.12.92.29] Basic mailspooler proxy authz attempted on non-Murder backend
Apr 21 15:10:24 nowhere.com cyrus/http[1653569]: auth failed - reinit