Sent emails "disappear"

When I send emails they show up in  my sent folder, but the recipients never get them. This is the 3rd time in the last 4 months that this has occurred. After a few days, everything starts working again and all is well for a while. But then without rhyme or reason, my emails stop sending again.  Very frustrating!!! I can't trust Comcast with my email anymore. I have talked to first level support and they have escalated to level 2. I have escalated twice and level 1 tells me that they have no idea when (or even if) they will ever get back to me. What kind of customer service is that? Can anyone help. I've tried all the suggestions that I have found in this forum and elsewhere, but nothing works.