There's no in-place way to move an existing mailbox from one domain to another — a mailbox is tied to the domain it was created on. Moving to a different domain means deleting the mailboxes on the old domain and creating new ones on the target domain.
What that means in practice:
The new mailboxes get new email addresses on the target domain — you can't keep the same local part (the part before the @) automatically moving to a new domain, since it's a fresh mailbox, not a transfer.
New mailboxes start with no warmup or sending history, so plan on warming them up for about 10–14 days before real outreach, the same as any newly created mailbox — see How warmup works and how to start it.
If the target domain is already connected to your Maildoso account, create the new mailboxes on it with Add More Email Accounts (using a free slot, or after adjusting your plan), then delete the old mailboxes — see Deleting, replacing, reducing, or cancelling mailboxes and domains for how deletion and slots work.
If the target domain isn't connected yet, connect it first — see Can I bring my own domains or domains I purchased already? — and wait for it to reach Active status (see Domain status glossary) before creating mailboxes on it.
Moving a large batch of mailboxes across domains, or wanting it coordinated with a renewal date? Contact support with the list and we'll help plan the timing.