Cannot send emails from Penang

[scldad]

I have been using roundcubemail for nine years and have happily accessed my mail server from all over the world.
However, I am now in Penang and can download incoming emails without issue but any attempt to send email gives an authorisation error.
I have tried connecting via my hotel WiFi and via mobile phone. No difference.
Nothing has changed at my server and nothing apart from location has changed on my Android devices  (three thereof).

PHP Error. Invalid response code from server.
Version 1.6.15. Rocky Linux server. Android clients.

HELP!!

[SKaero]

What response code is the browser receiving and can you very what the server is sending?

[scldad]

Unfortunately, Android browsers do not have (or I cannot find) a debug- facility where I can network traffic.
Here are the HTTP log entries:

219.93.12.138 - - [20/May/2026:10:13:51 +0930] "GET /roundcubemail/?_task=mail&_action=compose&_id=2885358406a0d02a429162 HTTP/1.1" 200 34475 "http://www.sdc.com.au/roundcubemail/?_task=mail&_mbox=INBOX" "Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36"
219.93.12.138 - - [20/May/2026:10:13:58 +0930] "POST /roundcubemail/?_task=mail&_action=autocomplete HTTP/1.1" 200 93 "http://www.sdc.com.au/roundcubemail/?_task=mail&_action=compose&_id=2885358406a0d02a429162" "Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36"
219.93.12.138 - - [20/May/2026:10:14:01 +0930] "POST /roundcubemail/?_task=mail&_action=autocomplete HTTP/1.1" 200 94 "http://www.sdc.com.au/roundcubemail/?_task=mail&_action=compose&_id=2885358406a0d02a429162" "Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36"
219.93.12.138 - - [20/May/2026:10:14:18 +0930] "POST /roundcubemail/?_task=mail&_unlock=loading1779237857109&_framed=1 HTTP/1.1" 200 398 "http://www.sdc.com.au/roundcubemail/?_task=mail&_action=compose&_id=2885358406a0d02a429162" "Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36"
219.93.12.138 - - [20/May/2026:10:16:06 +0930] "GET /roundcubemail/?_task=mail&_action=compose&_id=2885358406a0d02a429162 HTTP/1.1" 200 34475 "http://www.sdc.com.au/roundcubemail/?_task=mail&_mbox=INBOX" "Mozilla/5.0 (Linux; Android 10; K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36"