Try turning on the debug as explained a few posts up and maybe that will give us some clues as to what the issue is... POP is irrelevant here, I believe. POP/IMAP are for retrieving emails, but sending is done over SMTP either way. So if the problem is with sending, POP won't help...
Thread 0 > CamelFolder:get_message('INBOX', '16625') = class: CamelMimeMessage mime-type: multipart/alternative; boundary=94eb2c07ff807881310536bb767b content class: CamelMultipart content mime-type: multipart/alternative; boundary=94eb2c07ff807881310536bb767b class: CamelMimePart mime-type: text/plain; charset=UTF-8 content class: CamelDataWrapper content mime-type: text/plain; charset=UTF-8 class: CamelMimePart mime-type: text/html; charset=UTF-8 content class: CamelDataWrapper content mime-type: text/html; charset=UTF-8 < 0 > CamelException.setv((nil), 3, 'Host lookup failed: 64D602746BBA: Name or service not known') CamelException.setv((nil), 3, 'Host lookup failed: 64D602746BBA: Name or service not known') Thread 0 > CamelStore('maildir://localhost/home/user/.modest/outboxes/GmailID5'):get_folder('outbox', 0) = 0x4a2c0 < 0 > CamelException.setv((nil), 3, 'Host lookup failed: 64D602746BBA: Name or service not known') Thread 0 > CamelFolder:get_message('outbox', '1467619716.9341_3.64D602746BBA') = class: CamelMimeMessage mime-type: text/plain; charset=utf-8 content class: CamelDataWrapper content mime-type: text/plain; charset=utf-8 < 0 > CamelException.setv(0x438fed10, 2, 'Welcome response error: Unknown') Thread 0 > CamelStore('maildir://localhost/home/user/.modest/local_folders'):get_folder('sent', 0) = 0x333408 < 0 > sending : A00044 UID STORE 16625 FLAGS.SILENT (\Seen) received: A00044 OK Success