What are Flags?
Using Flags is a good way to highlight important messages and make them easy to spot in your mailbox. In Vivaldi Mail you can pick between 7 flag colors – red, orange, yellow, green, blue, purple and grey. You decide what each colour stands for.
Flags are synced with other mail clients that also support flags (e.g. Apple Mail). The red flag in some clients equals to starred messages (e.g. Gmail, Thunderbird).