Description Symfony Mailer selects a transport via the "MAILER_DSN" environment variable / configuration (e.g. "smtp://...", "sendmail://...", "native://default"). "SendmailTransport" invokes the local "sendmail" binary and supports two modes: "-bs" (speak SMTP over stdin: the default) and "-t" (read the message on stdin, pass recipients as command-line arguments). In "-t" mode, recipient addresses are appended to the sendmail command line without a "--" end-of-options separator. A recipient address beginning with "-" (which "Symfony\Component\Mime\Address" accepts as valid) is therefore interpreted by sendmail as a command-line option rather than an address. Resolution The "SendmailTransport" transport now ensure "--" is set before the list of recipients. The patch for this issue is available "here" (https://github.com/symfony/symfony/commit/c45144862dc289d03952f41f6078174089a3afc6) for branch 5.4. Credits Symfony would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix.