Postmark have confirmed issues with their API which they now believe it to be mitigated.

You can find more information at https://status.postmarkapp.com/notices/iejilmig3r9ldil5-increased-api-latency-timeouts

Update from Postmark:
Additional fixes have been deployed to remedy unhealthy API instances and are now recovering to their expected performance. We will continue to monitor before resolving this status.

Update from Postmark:
We are still experiencing latency issues with some API instances affecting a small subset of customers. The engineering team is continuing to investigate and remedy the issue. Updates will be provided as more information is available.

Update from Postmark:
A fix was deployed reducing latency and timeouts for unhealthy API instances. We are continuing to monitor while it recovers.

For customers using the built-in e-mail functionality (those using Office 365 are unaffected), our e-mail provider is currently experiencing API instability.

This may lead to delays in e-mails being sent, as we automatically retry to deliver your messages. We await further details from our e-mail provider (Postmark)