Email is actually designed with intermittent connectivity in mind. The sending server buffers outgoing emails until they're accepted by the receiving server. Typical settings are to retry every 4 hours, with a permanent failure being counted after 3-7 days.

Work's outbound email server for server/API/website emails (like password reset requests) has 127 emails in the queue, all to MS365 users.

Some of the big mail senders must currently have enormous queues of delayed messages.