Could not exe­cu­te: /usr/sbin/send­mail when using PHPMailer or sendmail in general

Today I ran across an issue I couldn’t make sense of in the first place. A contact form using PHPMailer couldn’t send mails any longer and came up with this error:

Could not exe­cu­te: /usr/sbin/send­mail

I wanted to send a quick test email by simply adding a new PHP file calling mail(). However I couldn’t FTP upload the file because the server rejected the upload.

This gave me a good hint: the partition on which the above directory sits was almost full (~ 96%) with just under 4 GB in total. So I enlarged this specific partition on the server and the sendmail error was gone.