They are suspected of sending a series of letters to 10 prisons that contained sheets of writing paper soaked in a solution of synthetic drugs such as mamba or spice.
The letters were intended for inmates, and it's believed they would tear off strips of the lined A4 paper to smoke.
But they were intercepted by prison security staff who noticed the letters, of which nearly 30 have been recovered, were discoloured and tinged around the edges.
Police linked the letters having spotted similarities of handwriting and common spelling mistakes which led to the raids being carried out.
Detective Inspector Tom Bowman, from the Prison Intelligence Unit at West Midlands Regional Organised Crime Unit said: “Tests have shown the sheets of paper had been soaked in a solution of water and synthetic cannabinoid before likely being oven-dried.
"The sheets of paper were concealed inside envelopes claiming to contain private and confidential material, such as legal documents, in the hope they wouldn’t be open to the same level of scrutiny as personal mail.
"The supply of drugs into prisons causes serious issue for the prison service in terms of the violence or health concerns of prisoners who take these substances."
The letters were sent between July 2019 and November 2021 to HMPs Birmingham, Hewell, Featherstone, Oakwood, Brinsford, Whitemoor, Ranby, Swinfen Hall and Lancaster Farms.
Prisons Minister Victoria Atkins said: “These arrests demonstrate the importance of our £100m investment in prison security measures – not just letter scanners but X-ray body scanners, drugs dogs, and other detection equipment.
“These all help us to clamp down on the drugs that fuel violence in our jails, and I commend our excellent prison staff and police colleagues for their vigilance in this case.”