mails only taking a single character for the recipient

A new install of FA 2.3.15 (created tonight). A working implementation of ssmtp so I can send both via the command line AND via the php mail() function (see below).

When I try and send an invoice by mail, the system responds with:

PURCHASE ORDER 1. Sending document by email failed. Email: s

Where the email is always the first letter of the company contact mail (i.e. the email is s if the mail address is ssss@example.com, and y if the email is ysss@example.com). Looking at the debug log from ssmtp, it is refusing the mail as it can't find a mailbox s@mydomain.com.

If I remove the mail contact, it says:

You have no email contact defined for this type of document for 'Supplier'.

If I have this as a .php file:

<?php
$to = "me@mydomain.com";
$subject = "Test mail";
$message = "Hello! This is a simple email message.";
$from = "someonelse@example.com";
$headers = "From:" . $from;
mail($to,$subject,$message,$headers);
echo "Mail Sent.";
?>

I get a mail from someonelse@example.com with headers/text as you'd expect - so the mail() function is working fine.

It looks like the mail is only taking the first letter of the name. Anyone any ideas?