Rather foolishly, I changed by domain name's MX record and then put it
back to how it was before. In each case I get "MX entry for mysite.com
has been changed to Bind reloading on [name of hosting company] using
rndc zone: new domain name.

It has been updated.

Now I cannot receive e-mail, but can send it - should MX changes happen
immediately or does it take a few hours? Thanks.

In news:441d84c6$0$19205$a82e6bb9@orc.meganetnews.com ,
Noone Here spewed forth:
It can take many hours. In fact, full Internet propagation could be up to 72
hours, according to my ISP.

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The sooner you fall behind, the more time you'll have to catch up.