I use koha testing docker in Mac OS. This code does not send a valid e-mail binary attachment: $pdf_binary_content = ...; my @attachments = ( { filename => 'attachment.pdf', type => 'application/pdf', content => $pdf_binary_content, } ); my $message_id = C4::Letters::EnqueueLetter({ ... ... ... attachments => \@attachments, }); because in C4::Letters is in _add_attachments function attachment encoding: ... Encode::encode( "UTF-8", $attachment->{content} ) ... I can't encode a binary file... Possible solution (C4::Letters): sub _add_attachments { ... foreach my $attachment ( @$attachments ) { $message->attach( # ################################################################## # Original code: Encode::encode( "UTF-8", $attachment->{content} ), # New code: defined $attachment->{binary} ? $attachment->{content} : Encode::encode( "UTF-8", $attachment->{content} ), # ################################################################## content_type => $attachment->{type} || 'application/octet-stream', name => $attachment->{filename}, disposition => 'attachment', ); } ... } my @attachments = ( { ... binary => 1, } ); my $message_id = C4::Letters::EnqueueLetter({ ... ... ... attachments => \@attachments, });