March 9 (Reuters) - Belgian postal company bpost on Tuesday forecast a 2021 core profit below market expectations, as it expects a strong volume decline in domestic mail throughout the year.

Bpost, which delivers mail and parcels and provides e-commerce logistics, sees adjusted earnings before interest and taxes (EBIT) in the range of 265 million to 295 million euros ($315.01 million to $350.67 million) in 2021, missing analysts' forecast of 300.8 million euros.

The group also expects total operating income to increase by a low single-digit percentage this year, seeing gross spending of between 200 million and 220 million euros.

As more shoppers turned online under extended coronavirus lockdowns, postal operators benefited from more parcel volumes even as traditional mail volumes have continued to tumble.

The firm, which operates in Europe, North America and Asia, expects its parcels and logistics units to grow in 2021, particularly in North America, while it flags a decline of 9% to 11% in underlying domestic mail volume.

The Brussels-based company posted an adjusted EBIT of 60.5 million euros in its fourth quarter, falling short of analysts' 74.5 million-euros estimate.

"Mail and retail was impacted by a second national COVID-19 lockdown in Belgium in November, resulting in pressure on mail volumes and proximity retail," Chief Executive Jean-Paul Van Avermaet said in a statement.

He noted the firm's parcels and logistics businesses continued to benefit from strong e-commerce development over the quarter.

Shares of Dutch rival PostNL, which rejected bpost's takeover offer back in 2016, last week jumped almost 4% after the company posted better-than-expected mid-term guidance, saying it will speed up its digital transformation to improve business performance, particularly in its parcels unit. ($1 = 0.8414 euro) (Reporting by Charles Regnier in Gdansk Editing by Jane Merriman and Matthew Lewis)