It has never happened to me in Poland. But maybe your conversation should go like this: "My e-mail is _not_ at gmail.com. It is: $firstname@$lastname.$tld. Ins

It has never happened to me in Poland. But maybe your conversation should go like this: "My e-mail is _not_ at gmail.com. It is: $firstname@$lastname.$tld. Instead of gmail dot com it is $lastname dot $tld." It would be tiring and a bit offensive to knowledgeable people, but maybe better then trying to debug the situation.