Actually, it's "the cheques are in the mail". Assuming there will be more than one cheque...
Sorry for the thread drift however this is a real bug bear of mine.
The word "however" doesn't work as a subordinating conjunction.
You really should say "Sorry for the thread drift, but this is a real bug bear of mine". You could use the word "however", but in this situation a semicolon or full stop should precede it.