I'm using msmtp to deliver system email from an Ubuntu server via an SMTP account.
Can I somehow configure this setup to rewrite all emails to one specific recipient address? If so, how?
Or put differently, I don't want any arbitrary PHP script or otherwise to be able to send email to anybody that is not me. I should be the only person that receives any email generated by any script on the server. All while only using msmtp (not using postfix or some other server).
I don't think this is possible with msmtp. From the man page (emphasis mine)
Msmtp transmits mails unaltered to the SMTP server, with the following exceptions:
- The Bcc header(s) will be removed. This behavior can be changed with the remove_bcc_headers command and --remove-bcc-headers option.
- A From header will be added if the mail does not have one. This can be changed with the set_from_header command and --set-from-header option. The header will use the envelope from address and optionally a full name set with the -F option.
- A Date header will be added if the mail does not have one. This can be changed with the set_date_header command and --set-date-header option.
- When undisclosed_recipients is set, the original To, Cc, and Bcc headers are removed and replaced with "To: undisclosed-recipients:;".
The undisclosed_recipients features was added recently. It does not appear to alter delivery though.
If you only cared about all local addresses being sent to a specific recipient (e.g. [email protected]) then you could use the aliases option with file contents
default: [email protected] 
set_from_header on: “‘on’ always sets a From header and overrides any existing one.” marlam.de/msmtp/msmtp.htmltoheaders?To:headers is not the default behaviour anyway. Just don't pass-t(--read-recipients) when invokingmsmtp.