Migration with the 'Only mail configuration and content' option is completed with the following error:
Error: domain "example.tld"
Failed deployment of mail user email@example.com
Execution of /usr/local/psa/admin/plib/api-cli/mail.php --create firstname.lastname@example.org -passwd '' -mailbox true -mbox_quota 209715200 -ignore-nonexistent-options failed with return code 1.
Unable to set the mailbox size limit for the domain: The size of this mailbox must not exceed the limit on amount of disk space allocated for mailboxes in this domain.
The subscription on the source server has #Mb 'Mailbox Size' value. After migration this value is set to be #Mb on the destination server according to the Default service plan.
There is no subscription with customized parameters on the destination server, that is why the value 'Mailbox Size' of the subscription is set according to the Default service plan.
Increase the value 'Mailbox size' of the domain on the destination server to required value and start new migration.
Also you can choose 'All configuration and content' option when creating a migration session. In this case the subscription will be migrated with all parameters according to subscription on the source server.