They still want their customers to be able to send emails to Russian servers. Having their servers straight up blocked would be annoying at best, disastrous for

They still want their customers to be able to send emails to Russian servers. Having their servers straight up blocked would be annoying at best, disastrous for some users at worst. Because email isn't like internet, the MXes connect directly and don't typically have intermediary relays. If Russia decided to straight up block all fastmail MXes - Russian customers wouldn't be able to transmit anything between Russian hosted MXes and Fastmail. So their actions protect their functionality, more than their potential customers.