Bottle Code

It's real simple. You put a message inna bottle, you put the bottle in some water, bada bing, bada boom, it'll get to your recipient.

— shady figure

The Bottle Code is a simple, yet strange, mechanism that is easily used by almost any individual living upon Istralar - so long as the fey have at some point swept through their area. It is a deceptively simple code: if something is placed in a bottle and released into a natural body of water with the intent of it reaching somewhere else, it will be whisked away by morning.
This is not to say it necessarily reaches its intended recipient - unless special steps are taken, there is only a chance that an intended bottle will find its way to a specific individual. As with all things fey, though, chance can be manipulated with care.
The Code applies internationally, and even across planar borders around the Inner Sphere. Whilst the azata of distant Elysium cannot send messages through bottles so easily without descending down to the Inner Sphere's planes, the efreeti of the Plane of Fire can certainly choose to do so (if they can find any water). Geniekind take great issue with the system - reportedly, many are accidentally bottled and sent hurtling across the planes, causing the stereotype of a 'genie in a bottle'.

Discovery

Our discovery or yours? Whose perspective is this from?

— a pixie lurking around the writer's shoulders

The fey seem to have begun their antics with glass bottles and messages in a time long prior to the spread of arcane magic, as Feidísfolk ruins in Ósksaelur Grotto feature a still-working pool designed for fey transportation of bottles. There, the grotto's wishing pool is frequently staffed by fey directly snatching the bottles up and fulfilling wishes with their own whims.

Genie's Bottle by Hanhula (via Midjourney)

Throughout the world, mortals took far longer to realise what was going on. Sending messages in bottles across the sea has been an ingrained facet of mortal myth in coastal areas for milennia, and yet literature delving into the Bottle Code is far more recent. It is possible that certain civilisations found this knowledge earlier, and simply kept it to themselves: notably, the ancient kingdom of Galasthin seemed disinterested in aiding research attempts when studies began in the third millennium EA.
Currently, research is ongoing on the potential of extending the fey magics powering the Bottle Code to the Outer Planes, where gods dwell. The centre for this research is located somewhere within the First World (where traditionally, direction and location matter little), but mortals are assisting from fey-dominant regions across Galasthin, Soniuch Zan, the Aletheian Empire, Fjolkandr, and other magic-heavy areas. This is assumed to be one of the largest cross-planar collaboration efforts, and if accomplished, would open new paths for other forms of planar communication or potentially even travel.

Twisting Chance

It's gotta be a strange twist of fate!

— giggling fairies

To convey a bottle to a specific person, place, or time, steps must be taken, and prices must be paid. The fey may be bought, but their favour must be bought in specific ways that change from day to day and season to season.

The most certain method of twisting chance is to seal the bottle in a second bottle, the outer one addressed to a specific known fey - even one of the demigods will do. This invariably draws the bottle to the attention of fey creatures, who snatch it up and investigate.
In this outer bottle, payment should be offered either directly (through gems, magical delights, and brilliant wildflowers) or indirectly (through promises of favours, written contracts, or stories). When accepted, the inner bottle will be set adrift unharmed, and the price will be paid for its safe travel. If it is not accepted, typically the bottle will be returned to the water, where another fey creature may investigate.
Those who would attempt this must be wary. The fey take their deals seriously. False gemstones, broken promises, or other lies might invoke terrible and spiteful reactions, with painful or potentially even fatal results. Fey are not mortals - they are far from it, and that should never be forgotten.

Offering by Hanhula (via Midjourney)

Parent Technologies

• Conjuration
• Divine Magic
• Leylines

Message in a Bottle by Hanhula (via Midjourney)

Inventor(s)

Unknown fey demigod(s)

Access & Availability

Accessible through any bottle & natural body of water

Complexity

Deceptively simple

Discovery

Pre-Worldrend

Related Species

• Fey

Magical Missive by Hanhula (via Midjourney)

I heard it even works on people if you get a big enough bottle! Maybe make sure you can breathe in there for that long, though...

— experimenter