Checkmk Werk 15456: mailbox: Fix exchangelib warning in the service summary

ID: 15456 Title: mailbox: Fix exchangelib warning in the service summary Component: Checks & agents Level: 1 Class: Bug fix Version: 2.3.0b1 Previously, a warning "WARNING:exchangelib.account:Failed to guess local timezone. Fallback to UTC" was shown in the mailbox service summary on some of the Checkmk servers.