A new era of messaging: Telegram - https://telegram.org/ was launched on August 14, 2013.
By name already old and well known but contemporary by content. Security and privacy are especially considered to be important. This is cloud-based environment, and forever free. Client source-code can be found from GitHub, server side by asking. More information: technical FAQ | regular FAQ and also from blog.
Time limits: conversations and files are kept forever in cloud and when not connected then joining later, all conversations and added files are visible.
limit: up to 1,5 GB per file, overall amount or volume of
files are not limited
If to delete the file by one participant then the file will be not deleted from cloud unless all participants delete it..
There is also search available and possibility directly propose development, improvement ideas to Telegram developers.
The program is available for all common software platforms, also web-based version exist. There are also portable versions available (no administrative rights needed) - just download, unpack and use.
The mobile phone number will be the username, no separate username will be not needed. The password will be confirmation code via SMS (or application if it was logged in or recently logged in somewhere else). Logging in is required once and if not logged out, it will keep logged in. Application can be protected with password and also two-step verification (password+SMS) is available whenever needed.
Allthough under settings there is possible later define a username, which in turn can be used to create a direct link like https://telegram.me/username - then there is no phone number needed to be shared in order to have a communication. It would be reasonable to use your real name - then it would be better understandable to who is involved.
Telegram is gradually replacing the e-mail as there are possible to send messages similarly interactively and can be read whenever there is time, opportunity for that. Also files can be sent in the same way as messages but much more larger than e-mail supports.
Sent messages can be edited - arrow key up or under mouse right button menu choose "Edit". To confirm message editing click the appropriate button in top right corner.
The groups can have up to 75,000 members each as stated at 25th January 2018. This number is subject of change in time. Channels are meant for one direction information broadcasting and can have unlimited number of subscribers.
More information: Groups, Supergroups, and Channels
Also voice messages can be sent - holding down the microphone button there will be sound file recorded directly into Telegram cloud and can be listened also from there.
All files (including pictures) can be browsed under the separate link from settings. The transmission of different files (including pictures) is extremely easy - just drag and drop to Telegram window. Pictures can be viewed in full screen and it works fast.
Also in smart devices Telegram would be easy to use - it will integrate with different programs and sharing e.g. pictures would be similarly easy as in desktop version. It would be even suggested to install Telegram at first in smart device - then it will sync contacts across devices and do not need enter them manually.
Contacts can be found/added by mobile phone number and if exist, also by
username. The username can be defined under settings after logging in via
mobile phone number.
PDF: Original download place | copy in EITC server
There can be used stickers to enrich conversations: