Background: I write PHP for fun, I don't compete with Sendy, I don't have any stake in any service that competes with Sendy. I also don't expect PHP to be written in the most OOP-everything Symfony-esque way either. But this is a bit too much for me even.
It doesn't follow any modern standards or guidelines; it doesn't use any templating system, just PHP hardcoded inline with HTML in hundreds of files (no modern framework, lightweight or not), concatenated variables into SQL queries and HTML. The author seems to hate using { } for if/else statements, which has the potential of introducing fun bugs if not extremely careful. 1 and 2 letter variable names are common (and I don't mean in the way of $i or $j); parts of it border on insane or bizarre; there are 500 character lines where instead of an error generation feature they just die() with a full HTML document as a string for error pages; functions are written inline all over the place and they all use the keyword global; there are no parameterised queries - it's covered in repetitive mysqli_real_escape_string and query concatenation. There is a massive amount of copy-pasted code: instead of some kind of single database management layer, there's a re-definition of a function that connects to a database with the same name in dozens of files, all of which try to read global variables (doesn't take any parameters)