Hi, Mohammad There are a number of reasons why this delay may be obtained and there are a lot of questions that may be asked. For example, is this a Common delay to all your receivers' domains? Or it is something only happening to a particular domain? This issue may also relate to your usage of the Mail API, if your Mail API quota was exhausted at the time. This is why I think it would be best for you to contact the GCP Support Engineers[1] on this issue, this way a GCP Support engineer would be able to work closely with you, reviewing your application configurations and determine exactly what may be the cause of the delay.
That being said, I gathered that the App Engine Mail API is deprecated and is not supported by newer runtimes and will not be in the future, so you may want to start considering a switch to other solutions at some point. [1]https://cloud.google.com/support-hub -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-appengine+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/6a76a38b-1d77-410d-910e-d8e1d925c28co%40googlegroups.com.
