Giving full mailbox access with auto mapping Exchange 365

 Firstly, launch an Exchange 365 powershell and connect to your tenant using connect-exopssession

FULL ACCESS (without AutoMapping): Add-MailboxPermission -Identity emailladdress@domain.co.uk -User emailladdress@domain.co.uk -AccessRights FullAccess -InheritanceType All -AutoMapping $false

SEND AS PERMISSION: Add-RecipientPermission emailladdress@domain.co.uk -AccessRights SendAs -Trustee emailladdress@domain.co.uk

Where -Identity is the account you want access to and -User / -Trustee is the account you want to allow access to it