First, as a conjunction, which it isn’t:
The Outside Directors and Officers Liability policy held by the firm on your behalf is set to expire on July 30, 2015, however your assistance is required in advance.
Don’t do this. It’s a bad run-on sentence, and just replacing the second comma with a semi-colon doesn’t fix it.
Segue to second misuse: at the beginning of a sentence (or after a semi-colon):
However, Gross Negligence or Wilful Misconduct does not include…
Perhaps not incorrect, but inelegant. Doesn’t that initial however somehow have a Valley Girl’s rising intonation to it? Not quite what you want in your client piece (or your contract, for that matter).
‘However’, properly used, is what is called a post-positive; it needs to rely on a previous statement, like so:
Mention was made, however, of a continued intention to implement…
Be careful, however, not to place it too early. It would be a bit weird to phrase that last example as ‘Mention, however, was made …’
Next writing tip: said, same, such