There is a blinking envelope icon on the display of my Nokia Symbian phone. Why?

The blinking closed envelope icon indicates the SIM card message memory is full.

Select Menu > Messaging > Options> (View folders > Options) >SIM messages and delete one or more SMS messages.

Note 1: Class 2 messages (for example, service messages from operators) are stored in the SIM card message memory. Once the SIM card message memory is full, Class 2 messages can't be received. 

Note 2: You have used a Nokia S40 product before. In S40 products the SIM card message memory is filled before the device message memory. If you insert a SIM card with full message memory to another device, a blinking message icon appears on the display of your Nokia device.

Note 3: Other reason could be that there is not enough free memory available on the device to store the new message. To release a space go to Messaging > Inbox and remove unnecessary messages.