The Empire has several ways you can engage in long distance communications.
Most communications, if not needed to be instant, are handled by the Imperial Postal Service. You can mail letters in any city or substantial town. Mail is more irregular in smaller communities. It's cheap but it's slow.
This involves sending your message via crystal ball. It's more expensive and several other people have to know what your message is, but you can get it to your target within hours... if he's in a city covered by the network.
The Chorus is a secret society of bard which passes messages through all sorts of magic and mundane means; they can get a message anywhere in a week.
A network of monks who pass messages by divine power or just running a lot between monasteries. Any monk or member of the clergy can use their services for free.
Many Dwarves know a code of hammer blows to pass messages; in the Underworld, this is used to send messages long distance by posts that hear and repeat the messages.
Druids have a network of intelligent trees that can move enough to make noises and pass messages like the dwarves, but less obviously.
More popular in earlier ages; too many people know how to hack them, but they remain used because communications is instant and you can have a conversation.
A patois of words, metaphors, jokes, idioms, and bits of languages used by thieves to communicate. It's also a network that links friendly thieves' guilds between cities and towns; only members can use the network.
Many magical and mundane courier services exist to haul your messages at reasonable rates.