You will have to send emails via your own smtp, so it will be auth.smtp.1and1.co.uk from here click here
The ' Relay access denied' is the clue it looks like you are trying to send it via freezone when you are on 1&1 they won't allow that.
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Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.