Until a few years ago I never had any problems, but during the last 3-4 years I had sporadic problems with messages that either were not delivered or they were delivered with a very large delay, e.g. a day, or they were delivered only if I resent them 3 or 4 times, at some of the large providers, like Gmail.

It still works fine most of the time, but when problems happen they are annoying because there is no way to know that something happened, unless I have an alternative way to ask the recipient, e.g. by phone.

In any case, now I always check that at least the final destination e-mail server has accepted the message, which should have been enough to guarantee the delivery, according to standards, but nonetheless this is not true any more.