If show_message() is not working, ensure the function is being executed correctly and debug/error reporting is enabled. Also check for redirects or output buffering that may prevent the message from displaying.

There is definitely something odd about this hotel.
I opened the server firewall for this IP then tried using K9Email.
I could receive but not send.
Then I fired up a VPN connection to my server and K9 could send (but not roundcube).
I'm home next week so will live with that for now.
Thanks for your help.

Thanks again for the great program.

Hi SKaereo, thanks for your reply. Yes, the tables were corrupted. I couldn't find a way to repair them—it didn't lead anywhere for me. I created a new database using the installer script instead. I did lose a bit of data, but nothing major.

It should since Roundcube is sending the email from the server its hosted on so the IP shouldn't be changing.

To change a PGP key password in Enigmail/Enigma, open your key management section, select the PGP key, then look for an option like "Change Passphrase" or "Edit Key." You will usually need to enter the current password before setting a new one.

It seems that my mailserver cannot reverse DNS lookup my hotel and that the hotel does not have a valid SPF record.
Could either of these affect roundcube?

Source of my issue is this commit: https://github.com/roundcube/roundcubemail/commit/374a4a5e25048f99cbded5be3fb7edec0605885f
It changes Mail_mime 'html_encoding' parameter from a fixed 'quoted-printable', to 8bit for most cases. Mail_mime does not wrap lines for 8bit, only when html_encoding is quoted-printable or base64.

I will add my finding to the original issue on Github as well.

It sounds like the database is corrupted, see if this fixes the issue: https://dba.stackexchange.com/questions/56849/innodb-tablespace-is-missing-for-table

That would point to the mail server doing something odd, maybe enable smtp_debug and post that log?