From my personal experience some 50 years and three or four wars for some poor sods ago in the days 'snail mail' and long before e-mail, Skye, and Sat phones, it can be defined alternatively by two words: 'joyful' and 'disappointing'. 'Joyful' when you get mail and especially so when there is a care package from a love one, and 'disappointing' when there is nothing from anyone, even family, for weeks or months.
Then, in the category of 'seen it', there were the 'Dear John' letters which usually arrived three to six months after lobbing up 'in country'. In that case, the letters were either enraging or crushing depending upon the recipient's personality and emotional investment in the relationship. Alternatively, though not any better, the young lady in question got bored, moved on to a new relationship and just quit writing.