Here's what memory alpha says:
As Starfleet did not learn of Voyager's circumstances until season 4's "Message in a Bottle", it is clear that they never did receive the crew's letters. Torres surmises at the end of the episode that, in spite of R'Mor's death prior to 2371, he may have passed Voyager's chip onto the Romulan Government.
In the noncanon Pocket novel series this is what happened:
In the 2350s, the Tal Shiar sponsored a number of experiments involving artificial wormholes and cloaking devices that utilised dark matter, that were performed by Doctor Telek R'Mor and Ambassador Lhiau. By 2356, the experiments were proved successful and Chairman Jekri Kaleh had thirteen D'deridex-class warbirds equipped with dark matter cloaking devices.
Following a presentation of the new technology to the Romulan Senate, the fleet of Romulan warbirds set out through an artificial wormhole to travel into the 2370s and capture the USS Voyager and bring it back to the 2350s, where they could use the starship's advanced technology. Thankfully, the Tal Shiar were unable to gain Voyager's advanced technology, but instead massed over 4,000 vessels along the Romulan Neutral Zone and prepared to attack five Federation colonies along the border, and using their dark matter cloaks and wormhole technology attack Earth, Vulcan, Bolarus IX, and Starbases 12, 74, and 212.
Thankfully, Lhiau and R'Mor were able to return to Romulus after escaping to Voyager and inform the Romulan Senate of the Tal Shiar's and Kaleh's plans to invade the Federation. The Empress was able to put an end to the assault, labelling the whole mission an exercise, and ordered that all dark matter technology be removed. With her great mission a failure, Kaleh retired from the Tal Shiar.
