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.
Google Workspace mailboxes: deleting one doesn't free its slot right away — it stays active until the end of the Google Workspace billing period. Create the new mailboxes first if you have unused Google Workspace slots; otherwise plan the move around the removal date. Never delete the domain's Admin mailbox while other mailboxes on it are still needed. See Deleting a Google Workspace mailbox.
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.