The Post Office by Augustus Pugin Senior and Thomas Rowlandson
for Ackermann's Microcosm of London (1808-11).
When my husband's great grandfather, who lived on the Rainy River in Koochiching Minnesota, decided to retire from full time farming, he opened a general store in Central Village and became the village's first postmaster.
Several of my indirect Seale line were Post Office employees. A few men from the Quebec line were postmasters, and cousin Henry Seale worked as an Assistant Inspector for the Post Office in Kingston, Ontario in 1918.
Look under Post Office in the local directories and almanacs.
The salaries of Canadian postal workers are in the Sessional Papers of the Dominion of Canada, under Auditor General Report. We find that Henry Edgar, as a Postal Inspector at Kingston, made $1350 for the year ending March 1919.
Lists of Unclaimed Letters at the Post Office were often posted in local newspapers, as well as the Government Gazette. Also check University websites, some have lists of unclaimed letters addressed to students who have graduated and moved out of the dorms without giving a forwarding address.
LAC - Post Office and Postmaster database search
Sessional Papers of the Dominion of Canada
Post Master finder - USPS
List of uncalled for letters, Wabash County, Indiana, 1861
List of letters unclaimed at Kingston, Ontario: Chronicle & Gazette 1833
Alphabetical list of Postmasters in Canada 1862
List of Post Offices in the United States, w/names of Postmasters etc 1828
Registered Letters of Canada: loss reported (missing money or contents) 1886
Registers of money, letters and packages sent to prisoners, US War Dept 1861-1865
Unclaimed Letters posted in the Victoria Government Gazette, AU
Allen County Genealogical Society Indiana - Unclaimed Letter Index
Unclaimed Letters at the Milwaukee Post Office 1836
List of Postmasters in Tasmania Newspaper 1853
The History of the Post Office Down to 1836 - UK
Canada Postal Guide 1821
Guide official du service postal Canadien 1917
Post Office Distribution Lists for Canadian Provinces
Newfoundland Post Office Circular (various years)
The Post Office of India and it's Story 1921
British Postal Museum -Appointment Records (available on Ancestry UK)
Unclaimed Letters at the Post Office, Auckland NZ to Dec 1862
Unclaimed Letters at the Post Office, Nelson, NZ 1857
Index to the Postal Working Map of Shanghai - 1904
Pre 1875 PEI Postmasters Database
Undelivered letters to Hudson's Bay Company men 1830-1857 (Google, limited view)
(Check at World Cat for libraries holding the above mentioned book)