Mac Mail re-indexing after reconnection to Exchange Mail server ?

When I move my MacBook between office and home location these days - I find something doing a massive read / write (Gigabytes ) - for several hours before Mail allows any incoming or outgoing messages to be transferred to/from Microsoft 365 Exchange server.

Is this Spotlight or Mail or both ? mds_stores / Mail / logd all showing 10's of gigabytes of reads under Disk in Activity Monitor. As soon (ie after hours) as all the disk Read/Write activity stops - the Mail messages start to flow......

Can't go on like this........(I know the Mails are there - can see them on other devices serviced by Microsoft Exchange server).