E-mail - Cannot send message from iPhone

Beginning 2-3 days ago, I cannot send an e-mail from my iPhone.  The e-mail remains in Outbox and I get a message:  "Cannot verify server identity" with sub-caption of "The identity of 'smtp.homestead.com' cannot be verified."  A "details" explanation points to a certificate authentication problem and uses the phrase "Not Trusted."  I AM ABLE to send and receive e-mails with my Windows-based laptop.  So this may be limited to Apple and / or iPhone products.  Please help.  Thank you!