MAILBOX TRANSPORT Submission service does one of the following actions BASED on the number and type of recipients.

For the message having only one mailbox recipient, it chooses a mailbox server in the target delivery group and give priority to the mailbox server based on the AD SITE proximity

With multiple mailbox recipients, it uses first 20 recipients to choose a Mailbox server in the closest delivery group, based on the AD site proximity

If there is no mailbox recipient, select a mailbox server in the LOCAL delivery group

Mailbox Transport Submission service does one of the following actions based on the number and type of recipients.

For the message having only one mailbox recipient, it chooses a mailbox server in the target delivery group and give priority to the mailbox server based on the AD site proximity

With multiple mailbox recipients, it uses first 20 recipients to choose a Mailbox server in the closest delivery group, based on the AD site proximity

If there is no mailbox recipient, select a mailbox server in the local delivery group