Reposted from a reply below:
On Sunday 2015-08-23 my outgoing email suddenly stopped working. Fortunately, my websites are on servers in Singapore, so I have prompt and competent technical support available. A few exchanges produced this reply:
Thank you for the information. From your test results it appears your Vodaphone USB modem is restricting port 25 but allowing port 587 either at the hardware modem level or Vodafone network level. The results indicates the issue is not with APC servers. If you wish to rectify the problem for your Vodafone modem there are two ways to do it.
1. Contact Vodafone to confirm if they are blocking outgoing smtp port 25. If yes can they allow it.
Or
2. Change your email program to use port 587 from port 25. Usually we expect Vodafone to reply you to use 587 as an alternative since if they block 25 most of the time 587 is allowed as an alternative. Your Vodafone telnet results indicates no issue with using port 587 as an alternative.