Parliament has since be re-opened.
The letter was addressed to Highlands & Islands Conservative MSP Jamie Halcro-Johnston and opened in his second-floor office by one of his researchers.
Mr Mountain said the invitation was to a Highland council event to mark the Inverness Courier's anniversary.
Another source told the paper that the "white powder" may have in fact have been imitation snow, however this has been denied by the police.
After around two hours after the discovery, police declared the incident resolved.
Tory MSPs Liz Smith, Jamie Halcro-Johnston, Edward Mountain were each sent suspect envelopes to their offices, according to the BBC.
"The whole block has been evacuated".
No details about the incident have been officially released but a Holyrood spokesman said it affected the MSPs' area of the building, and this had now been cleared of all MSPs and their staff.
Firefighters wearing protective gloves and helmets were testing the powder.More news: Chris Jericho to headline Japan's version of Wrestlemania
'No other areas of the campus are now affected and we remain open to the public at present.
"An incident has occurred requiring the evacuation of the MSP building at Holyrood", the parliament said in a statement. "Staff are working to assess the situation".
Police and fire services were on the scene as emergency procedures were activated and MSPs evacuated from the building.
'This is part of a pre-arranged process put in place, ready to deal with such developing situations'.
Part of the Scottish Parliament was evacuated after four suspicious packages were found.
The alarm was initially raised just after 11.30am when Police Scotland received a report of a suspicious package.
Police and firefighters were called, but business elsewhere in the parliament continued as normal.
A Scottish Parliament spokesman thanked "building users for their patience and co-operation".