Their 1977 expansion cousins, the Mariners, only ever had one home rainout.
On July 21, 1994, the Mariners came to Fenway Park for a 4-game series with the Red Sox. It was supposed to be played in Seattle at the Kingdome, but a few days earlier part of the ceiling collapsed before a scheduled game against the Orioles, which ended up being called off. Denied the use of BC Place in Vancouver as their temporary stadium, the Mariners played the rest of the season on the road.
The July 21 game against the Red Sox was supposed to be the return of the Mariners to play. However, it rained in Boston that day, and the game was moved to Saturday as part of a doubleheader.
If one still counts the transplanted games as home games for the Mariners, July 21, 1994 marked the only time that the team was rained out at home. The successor to the Kingdome, what is now T-Mobile Park, has a retractable roof. But other factors beyond the team's control can cause home games to be postponed.
Four of the Mariners games rescheduled in the wake of the 9/11 attacks in 2001, the same season where the Mariners won 116 games, but have not been back to the playoffs since, were a home series with the Rangers.
The Mariners in 2020 also had a home series with the A's postponed due to COVID-19 issues, and a home game against the Giants postponed due to air quality issues in the wake of a wildfire, five straight home games against the Giants and Padres were relocated to California.