- And what is your question for which you need an answer?
The description of your situation is short. It describes a challenge and provides also the root cause for addressing and solving it. So I don't see any question.
I don't know how old your application is which sends notifications via email. It does not seem to come with a builtin email (SMTP) server and seems to rely on existing email servers. Availability of such email servers may be assumed and often sufficient to just configure the interface for connecting to such email servers. And as email infrastructure has evolved over the past decades, the parameters of such interfaces became more and more numerous. And some parameters previously optional may have become mandatory in the mean-time. So your application needs to supply these parameters for contacting an email server and for using it for sending notification emails. When your application is too old, its previous updates may not yet have taken into account those changes in requirements on the interface to email servers to allow association to such email servers, and then your application simply needs an update of its interface to email servers in order to supply these mandatory parameters and eventually also optional parameters in order to continue using these previously working associations. There are just less and less email servers still allowing unauthenticated connections with their clients. Those few increase risk of getting abused for spam campaigns and as channels for transporting client attacks. These were also the reasons why many previously optional parameters became mandatory.
- Which authentication methods does your application currently support for connecting to email servers?
- Which authentication methods do your most common email servers support for connections of their supported email clients?