tbf bad grammar is not unusual when dealing with remote teams. The gmail is a bigger giveaway (but again, I've done business with big corporations via gmail addresses where it's just easier to use a personal address than get IT to sort remote access or a mailbox big enough to handle attachments...)

"We're mailing you a check, now please send the money to someone else" is always a red flag. In the rare cases someone doing this isn't scamming you, they're probably using you to avoid taxes or breach spending guidelines or something else unfavourable to you instead...