code: public MailAccount lookupUsedMailAccountForCurrentThread(final String accountReservationKey) {
  		lock.lock();
  		try {
  			Map<MailAccount, ThreadReservationKeyWrapper> usedAccountsForThread = filterValues(usedAccounts, new CurrentThreadWrapperPredicate());
  			if (!usedAccountsForThread.isEmpty()) {
  				for (Entry<MailAccount, ThreadReservationKeyWrapper> entry : usedAccountsForThread.entrySet()) {
  					if (entry.getValue().accountReservationKey.equals(accountReservationKey)) {
  						return entry.getKey();
  					}
  				}
  			}
  
  			return null;
  		} finally {
  			lock.unlock();
  		}
  	}
docs: Looks up the mail account that is reserved for the current thread under the specified key.
  
  @param accountReservationKey
  the reservation key
  @return the reserved account, or {@code null} if none is found