The maker of the Blackberry wireless e-mail device says it has restored the service to "most" of its North American users following a network failure.
Research In Motion said it was now looking at the cause of the breakdown, which first happened on Tuesday night.
Some US and Canadian users will still experience delays until the backlog of undelivered e-mails clears, it added.
But with eight million global users, some analysts have questioned whether the network has reached capacity.
"The rapid subscriber growth, plus the runaway junk e-mail boom, equals a disaster in the making," said technology expert Jeff Kagan.
"Networks work fine until they reach their capacity, then all sorts of strange things happen."
'Closely monitoring situation'
Canada-based Research In Motion (RIM) said it was "closely monitoring systems in order to maintain normal service levels".
It is estimated that RIM has around 45% of the market for smart phones.
Preliminary figures recently showed that the firm's profits jumped 10-fold to $187m (£94m) in the three months to 3 March.
The network disruption comes as RIM faces a formal probe by the US financial watchdog, the Securities and Exchange Commission, over its stock options.
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